Jeff Langman

Aquifer recharge is one of the most important hydrologic parameters for understanding available groundwater volumes and making sustainable the use of natural water by minimizing groundwater mining. In this framework, we reviewed and evaluated the efficacy of multiple methods to determine recharge in a flood basalt terrain that is restrictive to infiltration and percolation. In the South Fork of the Columbia River Plateau, recent research involving hydrologic tracers and groundwater modeling has revealed a snowmelt-dominated system. Here, recharge is occurring along the intersection of mountain-front alluvial systems and the extensive Miocene flood basalt layers that form a fractured basalt and interbedded sediment aquifer system. The most recent groundwater flow model of the basin was based on a large physio-chemical dataset acquired in laterally and vertically distinctive locations that refined the understanding of the intersection of the margin alluvium and the spatially variable basalt flows that filled the basin. Modelled effective recharge of 25 and 105 mm/year appears appropriate for the basin's plain and the mountain front, respectively. These values refine previous efforts on quantifying aquifer recharge based on Darcy's law, one-dimensional infiltration, zero-flux plane, chloride, storage, and mass-balance methods. Overall, the combination of isotopic hydrochemical data acquired in three dimensions and flow modelling efforts were needed to simultaneously determine groundwater dynamics, recharge pathways, and appropriate model parameter values in a primarily basalt terrain. This holistic approach to understanding recharge has assisted in conceptualizing the aquifer for resource managers that have struggled to understand aquifer dynamics and sustainable withdrawals.

A snowpack’s δ2H and δ18O values evolve with snowfall, sublimation, evaporation, and melt, which produces temporally variable snowpack, snowmelt, and runoff isotope signals. As a snowpack ages, the relatively depleted δ2H and δ18O values of snow will become less depleted with sublimation and evaporation, and the internal distribution of isotope signals is altered with melt moving through and out of the snowpack. An examination of δ2H and δ18O values for snowpack, snowmelt, and ephemeral creek water in the Palouse Range of northern Idaho indicated an evolution from variably depleted snowpack to enriched snowmelt and relatively consistent isotope signals in springtime ephemeral creeks. Within the primary snow band of the mountain range and during the winter–spring period of 2019–2020, the snowpack had an isotope range of −130 to −75‰ for δ2H and −18 to −10.5‰ for δ18O with resulting snowmelt values of −120 to −90‰ for δ2H and −16.5 to −12.5‰ for δ18O. With runoff of snowmelt to ephemeral creeks, the isotope values compressed to −107 to −104‰ for δ2H and −15.5 to −14.5‰ for δ18O. Aging of the snowpack produced increasing densities in the base, middle, and upper layers along with a corresponding enrichment of isotope values. The highest elevation site indicated the least enrichment of δ2H and δ18O in the snowpack base layer, and the lowest elevation site indicated the strongest enrichment of δ2H and δ18O in the snowpack base layer. Deuterium excess decreased with snowpack aging processes of accumulation and melt release, along with the migration of water vapor and snowmelt within the snowpack. It is likely that winter melt (early depleted signal) is a primary contributor to creeks and groundwater along the Palouse Range, but the strong variability of snowpack isotope signals provides a wide range of possible isotope signals to surface-water and groundwater systems at the mountain front.

The heterogeneity and anisotropy of fractured-rock aquifers, such as those in the Columbia River Basalt Province, present challenges for determining groundwater recharge. The entrance of recharge to the fractured-basalt and interbedded-sediment aquifer in the Palouse region of north-central Idaho is not well understood because of successive basalt flows that act as restrictive barriers. It was hypothesized that a primary recharge zone exists along the basin’s eastern margin at a mountain-front interface where eroded sediments form a more conductive zone for recharge. Potential source waters and groundwater were analyzed for δ18O and δ2H to discriminate recharge sources and pathways. Snowpack values ranged from −22 to −12‰ for δ18O and from −160 to −90‰ for δ2H and produced spring-time snowmelt ranging from −16.5 to −12‰ for δ18O and from −120 to −90‰ for δ2H. With the transition of snowmelt to spring-time ephemeral creeks, the isotope values compressed to −16 and −14‰ for δ18O and −110 and −105‰ for δ2H. A greater range of values was present for a perennial creek (−18 to −13.5‰ for δ18O and −125 to −98‰ for δ2H) and groundwater (−17.5 to −13‰ for δ18O and −132 to −105‰ for δ2H), which reflect a mixing of seasonal signals and the varying influence of vapor sources and sublimation/evaporation. Inverse modeling and the evaluation of matrix characteristics indicate conductive pathways associated with paleochannels and deeper pathways along the mountain-front interface. Depleted isotope signals indicate quicker infiltration and recharge pathways that were separate from, or had limited mixing with, more evaporated water that infiltrated after greater time/travel at the surface.

Abstract Equilibrium sorption and desorption experiments were conducted with clinoptilo-lite to evaluate the potential sorption/desorption of iron during different pH conditions. Sorption experiments indicated a partitioning of 0% to 17% of the iron in solution given pH of 2 to 4. The pH 2 solution was able to desorb 70% of the iron that was captured from a pH 3 solution. The largest desorption and sorption of iron and corresponding pH represent the end points of iron capture primarily by sorption/ exchange. These endpoints are the estimated pH pzc of 2.5 and the initial precipitation point of iron(II) at pH ~3.5. This acidity range is where clinoptilolite is able to capture iron without precipitation or the occurrence of full surface protonation. The inability of the highest acidity to remove all sorbed iron represents the greater bound iron that will not readily desorb with a change in pH. This retained iron creates a metastable state of the clinoptilolite that has a lower sorption capacity but reflects the ability of clinoptilolite to retain a sorbed transition metal with changes in pH. As pH varies, clinoptilolite may evolve in a sequence of metastable states reflective of its ability to capture or retain metals.

Practitioner points • Clinoptilolite is a capable reactive substrate, but its sorption/exchange effectiveness at low and variable pH and ability to retain captured metals was unknown. • Clinoptilolite retains its metal capture properties to a pH of 2.5 where surface proto-nation and mineral degradation likely occurs. • The ability of clinoptilolite to retain captured iron under greater acidity reflects an evolution of its sorption/retention capacity.

Passive treatment systems provide lower cost alternatives for remediation of mine drainage; however, acidic drainage increases treatment difficulty because of higher metal concentrations and proton competition for reactive substrates. A silica fiber functionalized with (3-aminopropyl) triethoxysilane (Si + APTES) and a naturally-occurring, microporous silicate mineral (clinoptilolite of the zeolite family) were evaluated in the laboratory as potential reactive substrates for passive treatment of mild (≥ pH of 3) acid rock drainage. Column permeability experiments with spun, 10-µm median diameter, silica fiber and loosely packed, 3.6-mm median diameter clinoptilolite indicate greater permeability and stability of clinoptilolite under flowing conditions. Batch sorption experiments with silica fiber (Si), Si + APTES, and clinoptilolite in a synthetic Fe(II)-SO 4 , pH 3.0 solution indicate an Fe specific sorption efficacy of Si + APTES > clinoptilolite > Si at equivalent surface areas. Specific sorption normalized to packing densities indicate greater sorption per volume for clinoptilolite. Sorption results for Si + APTES and clinoptilolite did not produce isotherms described by the Langmuir or Freundlich models, likely because of surface heterogeneity and precipitation reactions. Column sorption experiments under flowing conditions indicate an Fe removal efficacy of clinoptilolite > Si + APTES for permeable packing densities. Si + APTES demonstrated high specific sorption of Fe in batch sorption experiments and has potential use in low-flow, passive treatment of mildly acidic solutions. The balance of minimal surface preparation, greater permeability, structural stability, large surface area, micropore structure, and ion-exchange properties make clinoptilolite a better reactive substrate for passive treatment of mildly acidic solutions in high-or low-flow conditions.

The oxidation state of sulfur [S] is a primary control on mobility of metals in sediments impacted by legacy mining practices. Coeur d'Alene Lake of northern Idaho, USA, has been impacted by upstream legacy mining practices that deposited an estimated 75 Mt of metal(loid)-and S-rich sediments into the lake. Future lake conditions are expected to include algal blooms, which may alter S and metal remobilization during the seasonal euxinic environment. Cores of the lake sediments were exposed to anoxic and anoxic + algal detritus conditions for eight weeks at 4.5 • C through introduction of a N 2 atmosphere and addition of algal detritus. At a location 2.5 cm below the sediment-water interface, anoxic conditions promoted a shift in S species to continually larger concentrations of reduced species and an associated shift in the bonding environment reflective of increased S-metal bonds. Anoxic + algal detritus conditions suppressed the increasing trend of reduced S species and induced greater release of Mn compared to the anoxic-only conditions but did not appear to enhance the release of As, Cd, or Fe. The addition of algal detritus to the sediment-water interface of these Fe-and S-rich sediments enhanced mobilization of Mn likely because of dissimilatory metal reduction where the anaerobic oxidation of the algal detritus stimulated Mn reduction. Results of the study indicate that future metal release from the lake sediments will be altered with the likely deposition of algal detritus, but the effect may not enhance the release of acutely toxic metals, such as As or Cd, or substantially impact Fe cycling in the sediments.

Formation of dissolved metal particles (< 450 nm) in mining-impacted environments is a concern because of their potential for greater mobility and ecotoxicity compared to free ion and(or) sediment-bound metals. Metal-contaminated environments may produce soluble metal(loid) particles whose stability and transportability are determined by environmental conditions and particle composition. The Coeur d'Alene River Basin of northern Idaho, USA, is impacted by legacy mine waste-estimated 56 million tonnes of waste rock containing 900,000 t of Pb and 700,000 t of Zn were discharged into the Coeur d'Alene River and its tributaries during mining of argentiferous galena-sphalerite deposits. These legacy disposal practices resulted in substantial metal contamination including As, Cd, Fe, Pb, Mn, and Zn-of floodplain sediments. Monthly lakewater samples and sediment cores were collected along the shoreline of a metal-contaminated lateral lake of the Coeur d'Alene River. Porewater was extracted from upper and lower sediments to evaluate the formation and stability of dissolved metal particles during seasonal changes. Substantial concentrations of Fe, Pb, Mn, and Zn were present in 450-nm filtered porewater during each month, with variable increases and decreases of metal concentrations in filtered lakewater according to seasonal changes. Dissolved metal particles with an average diameter of 180 ± 115 nm were present in the porewater of the upper and lower sediments with size increases in early spring and decreases in fall. Particles in the lower sediment porewater were typically more stable, as indicated by more negative ζ potential values, and the greatest particle stability occurred during summer. Differences between upper and lower porewater metal particles correspond to changes in sediment S speciation and bond relocation resulting from an input of oxygenated groundwater. Transport of the dissolved metal particles in and from the sediments likely occurs with a change in the hydraulic gradient during a spring-to-summer transition that induces redox changes and increases particle stability. The presence of mining-related minerals and seasonal changes in environmental conditions allow for formation of dissolved metal particles, but the limited stability of the particles and/or low permeability of the sediments appear to limit, but not fully restrict, possible transport of metal particles to the overlying lakewater.

The efficacy of passive treatment systems for remediating acid rock drainage can be limited by the seasonal flux of discharge and metal concentrations that may not have been considered during treatment design. A review of passive treatment options for acid rock drainage indicates reduced efficacy due to seasonal periods of increased drainage and metal concentrations that lead to mineral precipitation, surface passivation, and flow bypass. In select cases, passive treatment systems prematurely failed due to seasonal flux or experienced substantially reduced treatment efficacy and life of the system. Complimentary systems are needed to minimise impacts from seasonal flux of drainage and metal concentrations to improve treatment efficacy and preserve the life of a multi-component system or a downstream primary system. Multi-component systems are possible with integration of existing treatment systems and design of new treatment options to tailor treatment to site specifications. ARTICLE HISTORY

At the Diavik Waste Rock Project's mine-research site, the microbial colonization and oxidation of waste rock sulfide minerals are attenuated by the extreme freeze-thaw cycle of a permafrost environment. The closure design for the waste rock stockpile consists of a low-sulfide waste rock and low-permeability till, covering a relatively higher sulfide waste rock. This design was examined at the mine site through construction of experimental waste rock piles and active zone lysimeters with and without the till cover. Leachate from these experiments indicates variable pH and SO 4 concentrations that correlate with sulfide content and the thermal moderating influence of the till cover. The till initially provided a moderated environment for the production of acid, growth of acidophilic Fe-and S-oxidizing bacteria, and enhanced weathering until wet up and freezing of the till and underlying waste rock as a permafrost. Greater sulfide oxidation was observed above the till cover because of greater exposure to the annual freeze-thaw cycle. An examination of the bacterial communities at the genus level indicates the prevalence of Pseudomonas, Rhodanobacter, Sideroxydans, and Thiobacillus in the waste rock. Pseudomonas spp. were dominant in the drier and more extreme temperature environment above the till cover, while Thiobacillus spp. were dominant in the more sulfide-rich, wetter/frozen environment below the till. A decreasing trend in Thiobacillus spp. from the exterior to the interior and an opposing trend in Acidithiobacillus spp. suggest greater acid generation deeper in the waste rock further from the extreme temperature variation of the tundra climate. The presence of the till cover moderated temperature variations, enhanced the initial rate of sulfide oxidation, and allowed for greater microbial diversity, but the freezing of the till cover and underlying waste rock drastically reduced sulfide oxidation and the generation of acid rock drainage. These results highlight the importance of temperature on microbially catalyzed acid production and our ability to use the extreme temperatures of the tundra climate to minimize potential environmental effects from mining through formation of waste rock permafrost.

Groundwater studies in the South Fork Palouse River Basin have been unable to determine recharge sources, subsystem connectivity and flow patterns due to the discontinuity of pathways in the heterogeneous and anisotropic aquifers located in Columbia River flood basalts and interbedded sediments. Major ion, δ 18 O, δ 2 H, δ 13 C, δ 34 S and temperature for groundwater collected from 28 wells of varying depths indicate a primary recharge source dominated by snowmelt along the eastern basin margin. This recharge can be separated into two distinct sources-a deeper and relatively less altered snowmelt signal (−17.3 to −16.8 δ 18 O, −131 to −127 δ 2 H, −12.9 to −10 δ 13 C, 18-23 • C) and a more altered signal likely derived from a shallower mixture of snowmelt, precipitation and surface water (−16.1 to −15.5 δ 18 O, −121 to −117 δ 2 H, −15.9 to −12.9 δ 13 C, 12-19 • C). A mixing of the shallow and deep source waters is observed within the upper aquifer of the Grande Ronde Formation near Moscow, Idaho, which results in a homogenization of isotope ratios and geochemistry for groundwater at nearly any depth to the west of this mixing zone. This homogenized signal is prevalent in a likely primary productive zone of an intermediate depth in the overall aquifer system.

Groundwater noble gas concentrations are used to model the temperature of aquifer recharge but also may be used to understand geologic influences on water chemistry. Aquifers in the South Fork Palouse River Basin are hosted within the fractured basalts of the Columbia River Basalt Group and the interbedded sediments of the Latah Formation. The travel time of groundwater, and the associated recharge rate, in this multiple aquifer system is of primary interest to water resource managers because of declining water levels. Prior investigations of the deeper groundwater indicated low values of percent modern carbon and elevated alkalinity and δ 13 C values compared to shallower groundwater. The groundwater travel time suggested by the uncorrected carbon-14 ages (up to 31,000 BP) implies a recharge rate much slower than hypothesized from identified recharge pathways, low storativity values, and relatively large groundwater withdrawals. No sources of dead carbon were previously identified, but the disconnect between old groundwater and likely recharge pathways suggest input from a dead carbon source. Groundwater collected for this study contained elevated alkalinity, δ 13 C, He, and 3 He/ 4 He values deeper in this aquifer system located in this young flood basalt province. Elevated alkalinity and δ 13 C values correlate with a mantle CO 2 source that is reflected in oversaturated He and elevated 3 He/ 4 He (R/ Ra) ratios. The flood basalts are Miocene expressions of the modern Yellowstone hotspot, which exudes relatively large concentrations of He (with elevated 3 He/ 4 He (R/Ra) ratios) and CO 2. A deep and 14 C-free geologic source of CO 2 helps to explain the low percent modern carbon in the deeper groundwater. Correction of groundwater ages through incorporation of this dead carbon source produced ages up to 64% smaller than previous age estimates; although, greater age corrections may be necessary because of differences in sample and analysis methods for alkalinity, δ 13 C, and 14 C that influence detection of the dead carbon input.

Formation of acid rock drainage is a temporal balance of acid-generating and alkalinity-generating reactions during mineral weathering. Accessory calcite and apatite are commonly present in granitoid rocks, and their weathering can be a source of net alkalinity. To evaluate the acid-consuming capability of the calcite and apatite group minerals in a granitoid waste rock, a humidity cell experiment was conducted with a mix of a high carbonate tonalite and a low carbonate pegmatite representative of the country rock surrounding a kimberlite pipe at the Diavik Diamond Mine in Canada. The alkalinity generated from the tonalite + pegmatite was evaluated through release of Ca and Sr, which indicated a primary alkalinity-generating period during the first 25 weeks of the 80-week experiment. This period represents decomposition of readily available carbonates (infillings) and phosphates (euhedral grains) that release acid-consuming CO 3 and PO 4 into solution. The total alkalinity-represented by the release of Ca-produced during the first 25 weeks was compared to the total acidity-represented by the release of SO 4-produced during the first 25 weeks of a previous humidity cell experiment with an acid-generating (Type III) waste rock from the same site. The easily weathered carbonates and phosphates of the tonalite + pegmatite have the potential to produce sufficient net alkalinity to consume the net acid generated from the early weathering of sulfides in the Type III waste rock. Aqueous extractions performed on post-experiment samples of the tonalite + pegmatite indicate remaining carbonate and phosphate minerals that were not readily available at the experimental ≤6.3 mm size. The heterogeneity of carbonate content and availability in granitic country rock at the mine site are reflective of the variability of accessory calcite in granitoids and indicate a continued need for site-specific determination of alkalinity-generating reactions with the weathering of waste rock.

The formation and transport of geogenic metal nanoparticles in mining-impacted environments is a developing concern because of their potential for greater distribution compared to larger particles. Discharge from the abandoned Gem Mine in the Coeur d'Alene Mining District of northern Idaho was examined for the presence of metal nanoparticles from weathering of an ore body of galena [PbS] and sphalerite [(Zn,Fe)S] with associated carbonate zones of siderite [FeCO 3 ] and ankerite [Ca(Fe,Mg,Mn)(CO 3) 2 ] in intruded quartz veins. Analysis of this circumneutral discharge from the abandoned mine and groundwater in the receiving shallow aquifer indicate poor-quality mine drainage containing nanophase Zn-CO 3 form(s) that dissociate into smaller particles or ions with release into the new geochemical environment of the aquifer. The nanoparticles were identified through acid titrations and dynamic light scattering analysis of 450-nm-filtered mine water. The stability of the nanoparticles was estimated through ζ potential analysis of mine water and groundwater, which indicated limited stability of the nanoparticles that was sufficient for transport in the mine drainage but insufficient for transport in the aquifer. The release of the nanophase Zn-CO 3 form(s) likely occurs through weathering of secondary carbonate minerals in the mined areas of the Gem-Gold Hunter deposit through water-rock interaction → crystal repulsion → particle detachment → solution entrainment → and limited dissociation during transport as opposed to crystal formation in solution with mineral-phase saturation.

The Diavik Waste Rock Project, located in a region of continuous permafrost in northern Canada, includes complementary field and laboratory experiments with the purpose of investigating scale-up techniques for the assessment of the geochemical evolution of mine waste rock at a large scale. As part of the Diavik project, medium-scale field experiments (∼1.5 m high active zone lysimeters) were conducted to assess the long term geochemical evolution and drainage of a low-sulfide waste rock under a relatively simple (i.e. constrained by the container) flow regime while exposed to atmospheric conditions. A conceptual model, including the most significant processes controlling the sulfide-mineral oxidation and weathering of the associated host minerals as observed in a laboratory humidity cell experiment, was developed as part of a previous modelling study. The current study investigated the efficacy of scaling the calibrated humidity cell model to simulate the geochemical evolution of the active zone lysimeter experiments. The humidity cell model was used to simulate the geo-chemical evolution of low-sulfide waste rock with S content of 0.053 wt.% and 0.035 wt.% (primarily pyrrhotite) in the active zone lysimeter experiments using the reactive transport code MIN3P. Water flow through the lysimeters was simulated using temporally variable infiltration estimated from precipitation measurements made within 200 m of the lysimeters. Flow parameters and physical properties determined during previous studies at Diavik were incorporated into the simulations to reproduce the flow regime. The geochemical evolution of the waste-rock system was simulated by adjustment of the sulfide-mineral content to reflect the values measured at the lysimeters. The temperature dependence of the geochemical system was considered using temperature measurements taken daily, adjacent to the lysimeters, to correct weathering rates according to the Arrhenius equation. The lysimeter simulations indicated that a model developed from simulations of laboratory humidity cell experiments, incorporating detailed representations of temporally variable temperature and water infiltration, can be scaled to provide a reasonable assessment of geochemical evolution of the medium-scale field experiments.

Alaska is already beginning to be affected by changes in global climate which make it a good location to study the feedback effects between climate, the water cycle and the carbon cycle. Using river dissolved elements and Sr isotopes we examine changes and/or differences in chemical weathering between watersheds in predominantly permafrost areas and glacial watersheds. Tributaries of the Tanana, Yukon, Nenana and Copper rivers were sampled during the early snow melt in late May and the late permafrost/glacial melt period in September of 2007. Waters are predominantly Ca– HCO 3-/SO 4 which is typical of glaciated terrains. 87 Sr/ 86 Sr isotopes indicate three potential end-members, young basalts, radiogenic silicates and marine carbonates. The results are consistent with weathering observed in glaciated regions with trace calcites and salts dominating the dissolved load; however we have evidence for silicate weathering. Results also indicate that permafrost watersheds experience more progressive silicate weathering than glacial watersheds. MOTIVATION, BACKGROUND AND SCOPE Geochemistry and isotope geochemistry of rivers are often used to understand hydrogeological processes in watersheds. For example, we can get a better understanding of the interaction between the carbon and hydrogeological cycles. An important issue that has recently received attention, and is partially motivating the proposed study, is calculating atmospheric CO 2 drawdown as a function of the type of silicate mineral being weathered. Weathering of certain types of silicates, especially calcium and magnesium silicates, are of interest because it removes atmospheric CO 2 through transport of bicarbonate to the ocean (Berner et al. 2003). The long-term carbon cycle is an important component of climate on a geologic time scale.

Existing δ 2 H and δ 18 O values for precipitation and surface water in the Nile Basin were used to analyze precipitation inputs and the influence of evaporation on the isotopic signal of the Nile River and its tributaries. The goal of the data analysis was to better understand basin processes that influence seasonal streamflow for the source waters of the Nile River, because climate and hydrologic models have continued to produce high uncertainty in the prediction of precipitation and streamflow in the Nile Basin. An evaluation of differences in precipitation δ 2 H and δ 18 O values through linear regression and distribution analysis indicate variation by region and season in the isotopic signal of precipitation across the Nile Basin. The White Nile Basin receives precipitation with a more depleted isotopic signal compared to the Blue Nile Basin. The hot temperatures of the Sahelian spring produce a greater evaporation signal in the precipitation isotope distribution compared to precipitation in the Sahara/Mediterranean region, which can be influenced by storms moving in from the Mediterranean Sea. The larger evaporative effect is reversed for the two regions in summer because of the cooling of the Sahel from inflow of Indian Ocean monsoon moisture that predominantly influences the climate of the Blue Nile Basin. The regional precipitation isotopic signals convey to each region's streamflow, which is further modified by additional evaporation according to the local climate. Isotope ratios for White Nile streamflow are significantly altered by evaporation in the Sudd, but this isotopic signal is minimized for streamflow in the Nile River during the winter, spring and summer seasons because of the flow dominance of the Blue Nile. During fall, the contribution from the White Nile may exceed that of the Blue Nile, and the heavier isotopic signal of the White Nile becomes apparent. The variation in climatic conditions of the Nile River Basin provides a means of identifying mechanistic processes through changes in isotope ratios of hydrogen and oxygen, which have utility for separating precipitation origin and the effect of evaporation during seasonal periods. The existing isotope record for precipitation and streamflow in the Nile Basin can be used to evaluate predicted streamflow in the Nile River from a changing climate that is expected to induce further changes in precipitation patterns across the Nile Basin.

Fractured-rock aquifers are inherently difficult for determining flow dynamics because of variability in fracture orientation and extension. A confined, fractured-rock aquifer in a semi-arid mountainous area of the Rio Grande Rift Zone was analysed for its response to recharge events that produced a pressure pulse within its potentiometric surface. The pulse was evaluated at the well scale and subaquifer level to evaluate flowpaths, travel times and dispersion and compare the bulk-scale aquifer response to possible velocities from slug test hydraulic conductivity measurements. Travel time and dispersion from the pulse proved comparable to probable travel times based on hydraulic conductivity measurements. Evaluation of the pressure pulse and the hydraulic conductivity measurements allowed for a holistic interpretation of the fractured-rock aquifer through analysis of two distinct data sets that provided corroborative evidence of flow dynamics and fracture connectivity. This holistic approach reduced uncertainty regarding the individual hydraulic conductivity values.

Identification of aquifer source waters can be difficult with traditional geochemical tracers such as solute concentrations because of the variability of rock-water interactions and complex recharge pathways. The growing use of traditional and non-traditional stable isotopes for identification of hydrologic processes allows the coupling of multiple stable isotopes and ion concentrations for cross-validation and better constraint of influences within an aquifer. A multi-isotope and age-dating study of ground water along the Western Caprock Escarpment of the Southern High Plains was implemented to identify saline-water intrusion and cross-formational flow in the Southern High Plains aquifer. This study coupled major ion and trace element concentrations with the stable isotopes of boron, strontium, hydrogen, and oxygen along with tritium-helium and carbon-14 age dating to identify potential source waters through differences in rock-water interactions and recharge pathways. Ground-water samples were collected from 16 wells, 13 of which were completed in the Ogallala Formation (primary formation of the Southern High Plains aquifer) and three that were completed in a minor aquifer of the underlying Dockum Group. Within the study area, a separate, local flow system originates at a topographic high and flows into a regional flow system defined by a large paleochannel. At a recharge area atop the topographic high and within the regional flow system, ground water is composed of a mixed cation-bicarbonate water, but a sodium-chloride type is present in parts of the local flow system and in ground water from the underlying aquifer in the Dockum Group. Major ion and trace element concentrations and the stable isotope composition of water (δ2H of -43.29 to -71.74 ‰ and δ18O of -5.85 to -9.95 ‰) do not indicate a simple two member mixing scenario but likely multiple sources and cross-formational flow between three formation deposits-- Permian salts, Dockum shales, and Ogallala alluvium. Boron (δ11B of 6.0 to 46.0 ‰) and strontium (0.70845 to 0.70906) stable isotope results helped to constrain the influence of rock-water interactions through different mineral signatures likely associated with the three formation deposits. The combining of various ion and isotope tracers allows the discrimination of source waters and the mixing strengths of the waters within the primary aquifer. Preliminary analysis suggests dissolution of Permian salts and migration of salt-laden water through fractures in the Dockum shales to a productive sand lens in the upper Dockum Group. Mixing within the Ogallala Formation and Dockum Group is likely occurring through upward and downward leakage between the two formations. Changes in the aquifers' potentiometric surfaces and available pathways control spatial mixing that produces a greater saline water contribution to parts of the local flow system that is diluted and (or) suppressed in the regional flow system of the Southern High Plains aquifer.

Understanding the links between climate change and water resources is becoming increasingly important both in terms of water quality and quantity. With Alaska likely to be more impacted than other regions, we sampled the central Alaskan rivers to determine its geochemistry and the biogeochemical processes that are active in the watershed. Using river dissolved load, we examine chemical weathering changes between watersheds that are in a predominantly permafrost area and those that are in glacial terrain. River water samples were collected on the tributaries of the Tanana, Yukon, Nenana and Copper rivers during two sampling events; one during the early snow melt in late May and the other during the late permafrost/glacial melting period in September. Our hypothesis is that the permafrost watersheds (the area between the Yukon River and Delta Junction) will show increased chemical weathering than the glacial-fed rivers and we are also likely to see evidence for redox cycling of the sulfur. Here we will use S and O isotopes of the river dissolved sulfate to determine the origin of sulfate. The possible processes include the dissolution of sulfate salts, anthropogenic inputs via precipitation, or redox cycling of pyrite or other sulfides within the watersheds. Previously, we presented 87Sr/86Sr isotopic values that ranged from 0.705, which is usually indicative of young basaltic source lithology, to 0.740, that suggests enriched silicate weathering. A number of rivers ranged between 0.709 and 0.711, suggesting carbonate or evaporite dissolution. Major ion results also indicate that the waters are predominantly Ca and SO4- . Ca concentrations range from 22 to 86mg/l while SO4- concentrations range from around 19 to around 140mg/l. The SO4- is associated with Ca rather than Mg and points to dissolution of gypsum. In glacial regions, the easily dissolvable minerals are expected to dominate the dissolved load. Preliminary isotope results on sulfate show a wide range of S (δ34S) isotope values from -7.3‰ to 12.2‰ and O (δ18O) isotope values from -8.4‰ to 14.4‰. The depleted S isotope values indicate possible redox cycling caused by depleted sulfides being re-oxidized to sulfate with minimal isotope fractionation. Positive values are consistent with gypsum dissolution though anthropogenic sources cannot be ruled out at this point. Negative O isotope values reflect influence of precipitation while the enriched values are indicative of other processes including evaporation. We will present a complete discussion of the different chemical weathering and sulfur cycling signatures between the different source regions and changes between the two melting periods.

Interest in global climate change has increased in recent years. Alaska is already beginning to be affected by the change which makes it a good location to study the impacts and feedback effects. Using river dissolved load we will examine changes and/or differences between chemical weathering of silicate rocks and the corresponding CO2 drawdown rates between watersheds that are in a predominantly permafrost area and glacial watersheds affected by tectonic activity. River water samples were collected on the tributaries of the Tanana, Yukon, Nenana and Copper rivers during two time periods, the early snow melt in late May and the late permafrost/glacial melt period in September. We hypothesize that with warming the permafrost dominated watersheds (the area between the Yukon River and Delta Junction) will show increased chemical weathering vs. glacial fed regions. While increased glacial melt could result in increased erosion and physical weathering the corresponding increase will not result in substantial increases in chemical load given the short time of concentration for some of these rivers. Estimating the amount of silicate weathering will not be trivial in this region given the complex lithology. The initial study uses Sr isotopes along with major ions to help identify and quantify the contributions from source lithology. The larger study includes B, S and O isotopes in a multi-tracer approach. The river waters have a composition of one-third Mg to two-thirds Ca which is typical of global rivers. The waters are predominantly Ca and SO4-. The Ca concentrations range from 22 to 86mg/l while SO4- concentrations range from around 19 to 140mg/l. The SO4- is associated with Ca rather than Mg and points to dissolution of gypsum. These results are consistent with work done by others in similar terrains where the easily dissolvable minerals dominate the dissolved load. Typically K and Si are contributed by silicate weathering and previous work has shown that glacial fed streams typically have more K and less Si. K concentrations range from about .440 to 2.4mg/l while Si ranges from 1 to 8mg/l. Sr concentrations range from 67ug/l to 400ug/l and preliminary 87/86 Sr isotopic values are around 0.705263, which is usually indicative of young basaltic source lithology. We will use Sr isotopes along with B isotopes to differentiate chemical weathering between the different source regions and also between early and late summer.

Page 1. Effects of Reservoir Installation, San Juan-Chama Project Water, and Reservoir Operations on Streamflow and Water Quality in the Rio Chama and Rio Grande, Northern and Central New Mexico, 1938-2000 By Jeff B. Langman and Scott K. Anderholm ...

Declining water levels in arid and semi-arid regions increase an aquifer’s vulnerability to natural and anthropogenic influences. A multi-isotope (δD, δ18O, 87Sr/86Sr, and δ11B) approach was used to resolve the geochemical evolution of groundwater in a declining aquifer in a semi-arid region of the southwestern USA as groundwater composition reacts to source-water mixing, cross-formational flow including saltwater intrusion, water–rock interaction, and likely agricultural recharge. Sub-aquifers or local flow systems are present in the Southern High Plains aquifer along the Western Caprock Escarpment in New Mexico, and the study site’s local flow system contains a Na–Cl, high dissolved-solids groundwater that flows from the escarpment until it mixes with a high quality regional aquifer or regional flow system. The local flow system contains water that is similar in composition to the underlying, upper Dockum Group aquifer. Saltwater found in the upper Dockum Group aquifer likely originates in the adjacent Pecos River Basin and crosses beneath or possibly through the hydrologic divide of the Western Caprock Escarpment. Strontium concentrations of 0.9–31 mg/L and a 87Sr/86Sr range of 0.70845–0.70906 were sufficient to estimate source-water fractions, mixing patterns, and contributions from chemical weathering through mass balance inverse calculations. Boron concentrations (59–1740 mg/L) and δ11B values (+6.0–+46.0‰) were used to confirm source-water mixing, further evaluate water–rock interaction, and examine the influence of possible agricultural recharge. Alteration of B concentrations and δ11B values in an area of likely agricultural recharge indicated the loss of B and decrease in δ11B values likely from plant uptake, adsorption, and weathering contributions in the soil/vadose zone prior to recharge. The effectiveness of 87Sr/86Sr and δ11B for resolving the geochemical influences in groundwater in the Southern High Plains along the Western Caprock Escarpment allowed for the reinterpretation of the isotopic composition of water that has been shown to be highly variable in the Southern High Plains. This study shows the utility of a multi-isotope approach for resolving the geochemical evolution of groundwater in an aquifer that has a complex relationship with underlying aquifers and the applicability of these isotopes as indicators of the alteration of source waters from natural or anthropogenic influences.

Lithology and geologic structure of a paleochannel in the Southern High Plains aquifer were tested for possible influence on source water mixing that has been implicated in the alteration of water types within the Southern High Plains aquifer. Geochemical and physical data suggested changes to source-water mixing across short distances in the Southern High Plains aquifer as a result of