Environ Monit Assess (2018) 190:604 https://doi.org/10.1007/s10661-018-6976-9 Reservoir water quality: a case from Jordan Ahmed A. Al-Taani & Nazem M. El-Radaideh & Wesam M. Al Khateeb & Abeer Al Bsoul Received: 22 December 2017 / Accepted: 13 September 2018 # Springer Nature Switzerland AG 2018 Abstract Jordan relies heavily on reservoirs building and development to cope with water supply challenges, where monitoring and assessment of reservoir water quality are critically important for the sustainable use of these water supplies. Mujib Dam is an important water supply source in central western Jordan. Evaluation of water quality parameters and their spatial distributions (vertical and horizontal) showed near-neutral pH values with nearly similar values from surface to bottom. The vertical profile of DO and TDS in the dammed reservoir showed slight decreasing trends with increasing depth. Although Ca, Mg, Na, and K concentrations varied slightly with depths, their variations showed no trends. Similarly, the vertical and horizontal distribution patterns of Cl, SO4, HCO3, NO3, and PO4 in Mujib reservoir water showed insignificant variations in surface water layer and relatively unchanged values or decreasing trends through the water column. Higher values of TN have been observed, especially in the western part, suggesting that agricultural activities and livestock farming in the upstream catchment are impacting water quality. Results revealed that weathering and dissolution of rocks are the major source of water chemistry. The majority of trace metal levels (Cd, Cr, Cu, Fe, Mn, Pb, Zn, Co, Ni, Sr, and B) in water showed relatively similar surface and bottom values. The concentrations of COD and BOD5 in surface water were relatively low with higher concentrations observed in the northwestern corner, coincided with higher levels of chlorophyll a. The average ratio of TN to TP in surface water suggests that phosphorus is the limiting factor for the algal blooms, whereas the average chlorophyll a level in surface water indicates oligomesotrophic water. A. A. Al-Taani (*) : N. M. El-Radaideh Department of Earth and Environmental Sciences, Yarmouk University, Irbid 21163, Jordan e-mail: taaniun@hotmail.com W. M. Al Khateeb Department of Biological Sciences, Yarmouk University, Irbid 21163, Jordan e-mail: wesamyu@gmail.com N. M. El-Radaideh e-mail: nazemelradaideh@yahoo.com A. Al Bsoul Department of Chemical Engineering, Al-Balqa Applied University, Salt, Jordan e-mail: abeermahmod@yahoo.com Keywords Mujib Dam . Spatial variation . Water quality . Drinking water . Jordan Introduction The inherent scarcity of water resources in Jordan is further complicated by the competing demands of agriculture and a rapid population growth (Al-Taani 2013, 604 Page 2 of 15 2014; Al-Taani et al. 2012). Jordan has recently received unprecedented waves of Syrian refugees, and the population number has doubled. This has placed a substantially growing burden on water resources. With its limited water resources, Jordan relies heavily on new reservoirs building and development to cope with water supply challenges (Al-Rawabdeh et al. 2014; Al-Taani et al. 2018). While water reservoirs are highly expensive, they have become an important water supply strategy for Jordan (Al-Rawabdeh et al. 2013), where dams supply about 15% of the annual water consumption (the total annual water need of about 971 MCM). Therefore, monitoring and assessment of reservoir water quality are critically important for the sustainable use of these water supplies (Al-Taani et al. 2015). Mujib Dam is a multipurpose water source which was built in 2003 in central western Jordan. The sustainability of Mujib Dam is challenged by the high sedimentation and subsequent loss of storage capacity (El-Radaideh et al. 2014, 2017a, b). In addition to the sedimentation problem, evidence of contamination from human activities has been recently observed (AlMalahmeh 2006; Hailat and Manasreh 2008; Manasreh et al. 2010), though the Mujib watershed is sparsely inhibited. In response to the growing risk of contamination, the Jordan Valley Authority (JVA) has designated a variety of protection zones to improve the quality of the drinking water provided by the Mujib Dam and enhance land-use planning in the catchment. A number of landuse restrictions were proposed by JVA to reduce contamination risks, especially those pertaining to agricultural management, livestock farming, and mining. This research focuses on evaluating the current status and spatial distribution (vertically and horizontally) of physical, chemical, and biological characteristics of Mujib Dam waters. This assessment is likely to unveil the natural and anthropogenic sources impacting the reservoir water chemistry. Also, it will help to maintain high-quality waters for the sustainable development and prosperity of the region in such a highly scarce-water country. Site description and geology Mujib Dam was built in 2003 in central western Jordan and intended to impound flood and base flows from Wadi Mujib to be used for irrigation, industrial and Environ Monit Assess (2018) 190:604 domestic (including drinking water) purposes, which otherwise are discharged into the Dead Sea. The reservoir is 5 km long, with a maximum width of 1100 m and a maximum water level of 194 m at full capacity (Ijam and Al-Mahamid 2012). With a surface area of about 1.98 km2, the dam’s maximum storage capacity is about 31.2 MCM (live storage is about 29.8 MCM, dead storage is 1.4 MCM, and an annual yield of 16.6 MCM). It is founded in the limestone of deeply incised valleys with roller compacted concrete gravity. Mujib catchment (of about 4500 km2) is drained by several wadis forming the base flow of Mujib Dam and originates primarily from the continuous array of springs within the Dead Sea escarpment, and partly from the Ajloun Series and the lower sandy aquifer systems (NWMP 2004). These wadis slope gently along the Jordan Highland and Plateau (of 700–900 m above sea level) before plunging down as they approach the eastern escarpment of the Jordan Rift Valley and flow to the Dead Sea (of 410 m below sea level). Mujib Dam is located in a semi-arid to arid climate, with cold and rainy winters, and hot dry summers with frequent drought conditions. The annual rainfall varied spatially from about 300 mm in the northwestern corner to less than 50 mm in the southeastern part of the watershed. Most precipitation occurs as intense storms in the winter months extending from October to April. The average annual potential (pan) evaporation is 2200 mm. The Mujib catchment, which is part of the larger Mujib basin (consisting of Mujib and Walla catchments), is covered by sedimentary rocks of Mesozoic and Cenozoic with minor occurrence of basic intrusions. The rock units, exposed in the catchment, are of Lower Cretaceous to Recent age. Umm Rijam formation, which is the youngest formation, is exclusively exposed in the eastern watershed and consists primarily of cherts and limestones. This formation is underlain by Muwaqqar formation aquiclude of chalky marls. Amman-Umm Ghudran formations are cropped out in the central catchment and consist mainly of silicified limestones, cherts, chalky marls, and chalks. Ajlun Group which crops out in the western part consists largely of a thick sequence of alternating limestone, siltstone, marlstone, nodular limestone, and shales. Kurnub Sandstone Group exposed in western area is mainly composed of medium- to coarse-grained Environ Monit Assess (2018) 190:604 sandstone with some intercalations of siltstones and clay stones. Materials and methods Water sampling was primarily conducted in July 2015 (mid-summer season) and 60 water samples were obtained. Sampling sites collected for the Mujib Dam are presented in Fig. 1. Three water samples were collected from each sampling points at varying depths from surface to bottom using water depth sampler. One liter of unfiltered water samples at each site were collected in Page 3 of 15 604 pre-acid-cleaned polyethylene containers and preserved at 4 °C for 24 h. Following collection, a split of two samples analyzed and the results were averaged. Water samples were subjected to a variety of physical, chemical, and biological analyses. Ca, Mg, Na, K, Cl, SO4, HCO 3 , TDS (total dissolved solids), TSS (total suspended solids), TH (total hardness), NO3, NO2, NH4, TN, PO4, TP, BOD (biological oxygen demand) (5-day BOD Test (Ref. WW-BOD5-R003)), COD (chemical oxygen demand) (Closed Relux Titration Method (Ref. WW-COD-R005)), Chl a (chlorophyll a) (Fluorometric Determination (Ref: MIC-CHA-R*003)), pH, temperature, and DO (dissolved oxygen) were Fig. 1 Location map of water sampling points selected from Mujib Dam 604 Page 4 of 15 analyzed. Water temperature, pH, DO, and TDS were measured in situ using portable meters after calibration. Measurements and analyses were performed according to standard methods (APHA 1998). In addition, Cd, Cr, Cu, Fe, Mn, Pb, Zn, Co, Ni, Sr, and B were also analyzed. The average ionic mass balance calculated was 2.44%. For quality control, triplicates of each sample were analyzed, and average values were calculated. Accuracy of the atomic absorption spectrometer analysis was checked using National Institute of Standards and Technology standard reference material SRM 1646 (estuarine sediment). The spatial distribution maps were created using the software Arc GIS 9. Results and discussion A GIS-based map was produced to present the spatial distribution of water depths based on our field survey and measurements (Fig. 2), where the maximum depth recorded was 30 m (closer to the dam’s wall) and a minimum depth of 10 m with an average of 19.7 m. Environ Monit Assess (2018) 190:604 Spatial distribution patterns in temperature, pH, and dissolved oxygen The pH values exhibited near-neutral water. The surface pH value ranged between 7 and 8.1 with an average value of 7.7, whereas in the bottom water layer, the pH levels varied from 7 to 7.6 with a mean value of 7.3 (Table 1). The spatial distribution of pH values showed that lower values occurred closer to the dam’s wall and the higher values were found in the southern reservoir. The relative low levels of pH in surface water observed in the northwest (adjacent to the dam’s wall) are likely related to decomposition of algal mass and other flora in this part of the reservoir (Al-Taani 2013, 2014). DO concentrations varied from 1.4 to 6.3 mg/l and 1 to 3.8 mg/l in surface and near-bottom water layer, respectively (Table 1). The surface fluctuation of DO values showed that lower values occurred in the northwestern reservoir and relatively higher levels in the southern segment of the dam (Fig. 3). The pH surface distribution is consistent with the relatively lower dissolved oxygen contents (Fig. 3) (and TN concentrations (Fig. 9) which are mainly organic nitrogen) observed in this portion of the dam’s water and is likely related to decomposition of algal mass. Higher pH values were observed in the winter season (Manasreh et al. 2010), primarily due to leached alkaline ions from weathering of carbonaceous deposits in the catchment. However, in the end of algal bloom period, decomposition of dead algae and other flora becomes widespread and pH decreases. Although slightly higher rates of photosynthesis were measured in the northwestern part (Fig. 10), the overall Table 1 Basic statistic of water quality parameters for surface and bottom samples collected from Mujib Dam Sample Surface Bottom Fig. 2 Depth distribution map of Mujib reservoir bottom pH Temp. DO Min 7 22.5 1.4 Max 8.1 26.6 6.3 Mean 7.7 25.3 3.8 SD 0.38 0.95 1.15 Min 7 13 Max 7.6 19.3 3.8 Mean 7.3 15.3 2.6 SD 0.17 1.85 1 0.85 Environ Monit Assess (2018) 190:604 Page 5 of 15 604 Fig. 3 Spatial variations in pH and DO levels in surface water of Mujib Dam rates remain relatively low (compared to other dams in Jordan (Al-Taani 2013, 2014)). This suggests that the organic matter contents (resulting from decaying of dead algae) in water is also low. As explained later, eutrophication process is limited in Mujib reservoir, resulting in a decreased content of organic matter in the near-bottom waters as well. However, the average pH levels in water column showed nearly similar values from surface to bottom (Fig. 4). These values of pH are not likely to contribute to nutrients and trace metals release from the surficial bottom sediment. This is also consistent with the low concentrations of trace metals in the near-bottom water layer (Table 4). The vertical profile of DO in the dammed reservoir showed a slight decreasing trend with increasing depth (Fig. 3). The average surface DO ranged between 1.4 and 6.3 mg/l decreasing vertically to values that varied from 1 to 3.8 near the bottom (Table 1 and Fig. 3). These results suggest that the relatively shallow water of Mujib reservoir may have been probably mixed and that lake was not stratified, during water sampling. Fig. 4 Vertical profiles of DO, pH, and temperature levels (left) and TDS concentrations (mg/l) (right) in water samples collected from Mujib Dam 604 Page 6 of 15 Environ Monit Assess (2018) 190:604 In surface water layer, the total suspended solids (TSS) varied from about 2 to 12.4 mg/l with a mean value of 5.6 mg/l. Though TSS was not analyzed in the near-bottom water layer, it is likely that TSS decreased downward. With some exception, Fig. 5 showed that TSS values are relatively equally distributed throughout the surface water layer, suggesting that TSS is not primarily resulted from algal growth and decay. Water use and pumping during the summer period in response to increased use of water is probably the main factor influencing TSS levels (Al-Taani 2013). Alkalinity in Mujib reservoir water averaged 100 mg/ l and about 118 mg/l in surface and bottom water, respectively (Table 2), indicating a relatively high buffering capacity and a tendency for the reservoir to exhibit nearly stable pH. The concentrations of Ca, Mg, Na, and K varied from 40 to 59.9 mg/l, 12.81 to 24.05 mg/l, 31.61 to 42.60 mg/ l, and 5.77 to 8.79 mg/l in surface layer, respectively (Table 2). Their spatial fluctuation patterns showed almost dissimilar distribution with no distinct horizontal trends (Fig. 6). However, these ions ranged from 34 to 55.4, 11 to 19.1, 27.7 to 37.3, and 6.1 to 8.7 mg/l for Ca, Mg, Na, and K, respectively, near the reservoir bottom (Table 2). Although Ca, Mg, Na, and K concentrations varied slightly with depths, their variations showed no trends (Fig. 7). However, they appear to show increased concentrations closer to the reservoir bottom similar to that of TDS and probably related to re-suspension of precipitated salts (Batayneh et al. 2014b). The concentrations of Cl, SO4, and HCO3 varied from 40.5 to 46.5, 89 to 95.3, and 132.2 to 161.2 mg/l in the reservoir surface water, respectively, whereas in The surface temperature of water ranged between 22.5 and 26.6 °C, whereas in near-bottom water, it fluctuated from 13 to 19.3 °C (Table 1). In contrast to pH and DO, the average temperature values decrease sharply through the water column. Lower temperature in the bottom water layer coupled with the effect of neutral pH are probable causes for the lower activities of decomposer in this water layer which is mirrored in the relatively unchanged DO levels. Spatial variations in dissolved ions While the spatial distribution map of total dissolved solids (TDS) showed relatively similar values throughout the surface water layer, they vertically varied. TDS contents in the surface water layer ranged between 400 and 577 mg/l with an average value of 495 mg/l (Table 2) with nearly equally distributed values across the reservoir water (Fig. 5). However, the TDS showed lower values near the reservoir bottom ranging between 330 and 476 mg/l with a mean value of 400 mg/l at the base. The apparent vertical variation in TDS with higher values at the surface and lower levels near the bottom (Fig. 4) is probably related to higher temperature of surface water layer that increased water evaporation rate with subsequent increase in TDS compared to that in the deeper layer (Batayneh et al. 2014a). Figure 4 also showed that TDS values declined with depth with apparently slight increases at the bottom. This is probably attributable to water-sediment interaction at the reservoir base allowing re-suspension of precipitated salts near the bottom water (Batayneh et al. 2014b). Table 2 Summary statistics of water quality data (mg/l) for surface and bottom samples collected from Mujib Dam Sample Surface TDS TSS Min 400 2 40 12.81 31.61 Max 577 12.40 130.68 59.9 24.05 42.60 Mean 495 5.6 100.2 51.4 17.8 37.5 6.6 30 3.39 SD Bottom Total alkalinity 87.12 11.66 Ca 6.17 Mg 3.17 Na K HCO3 Cl SO4 5.77 40.51 89 132.2 8.79 46.45 95.29 161.17 44.1 92.5 145.3 3.61 0.99 1.42 1.25 6.58 Min 330 – 91.96 34 11 27.68 6.12 27.46 59.62 Max 476 – 179.08 55.38 19.1 37.3 8.68 45.32 95.83 151.17 Mean 400 – 117.85 47.3 13.7 32.6 7.4 35.0 73.5 134.9 41 – 20.37 3.0 0.54 SD 5.91 2.36 5.04 10.23 122 7.74 Environ Monit Assess (2018) 190:604 Page 7 of 15 604 Fig. 5 Spatial distribution of TDS and TSS (mg/l) in surface water Mujib Dam near-bottom water, they fluctuated from 27.5 to 45.3, 59.6 to 95.8, and 122 to 151.2 mg/l, respectively (Table 2). Similar to the major cations, the spatial distributions of Cl, SO4, and HCO3 in surface water showed relatively no specific patterns, with SO4 and HCO3 exhibiting nearly equally distributed values (Fig. 8). The vertical profiles of Cl, SO4, and HCO3 in water column are presented in Fig. 7. Although the majority of Cl, SO4, and HCO3 concentrations varied with depth, their variations showed slightly decreasing trends. Likewise, the bottom values show relatively slight increase and may be related to re-suspension of sediments. These results suggest that water is not stratified. TDS is strongly correlated with Mg, Cl, SO4, and HCO3 with r = 0.61, 0.80, 0.80, and 0.58, respectively (Table 6). However, weak correlations were observed between TDS and Ca and K, with r = 0.36 and 0.24, respectively. These results suggest that Mg, Cl, SO4, and HCO3 account for the majority of TDS content in water. Low NO3 concentrations were observed in surface water layer, ranging between 0.51 and 4.34 mg/l with an average of 1.4 mg/l. The vertical and horizontal distribution patterns of NO3 in Mujib reservoir water showed Fig. 6 Spatial distribution of Ca, Mg, Na, and K (mg/l) concentrations in surface water Mujib Dam 604 Page 8 of 15 Environ Monit Assess (2018) 190:604 Fig. 7 Vertical profiles of the concentrations (mg/l) of Ca, Mg, Na, K (left), Cl, SO4, HCO3 (middle), and NO3 (right) in water samples collected from Mujib Dam insignificant variations in surface water layer (Fig. 9) and relatively unchanged values through the water column (Fig. 7). This vertical trend suggests that the denitrification process in the reservoir water, particularly near the bottom, is insignificant and that water column was vertically mixed and relatively well-oxygenated (Fig. 4) (AlTaani et al. 2013). These low values of NO3 suggest that the water of Mujib Dam is not potentially impacted by human activities. Total nitrogen (TN) concentrations ranged between 1.73 and 7.73 mg/l (averaging 4.9 mg/l) in surface water layer (Table 3). Agricultural and wastewater discharges are probable non-point source pollution of TN. While Mujib Dam area has been designated as a protected region (as explained earlier) with a number of land-use restrictions, TN levels suggest that agricultural activities and livestock farming in the upstream catchment are impacting water quality. Unlike NO3, the spatial changes in TN values showed higher values adjacent to the dam’s wall (Fig. 9). These variations in surface distribution patterns of TN and NO3 suggest that NO3 contents probably account for a minor portion of TN and that the organic nitrogen is the primary constituent of TN. Low concentrations of NO2− and NH4+ were observed ranging between 0.004 and 1.003 mg/l and between 0.01 and 0.274 mg/l in upper water layer, respectively (Table 3). The relative lower levels of DO are probably partially responsible for the NH4 levels, where the relatively less oxic conditions promote the denitrification process and formation of ammonia compared to the oxygenated water. High pH levels can also cause conversion of NH4 to the toxic unionized form (NH3) (Al-Taani and Al-Qudah 2013; Abdelhay et al. 2018). Low levels of PO4 in surface water varied from 0.02 to 0.61 mg/l with an average of 0.2 mg/l (Table 3). The spatial distribution trends of PO4 in water showed insignificant vertical variations. Similarly, low concentrations of total phosphorus (TP) were found in surface water layer ranging between Fig. 8 Spatial distribution of Cl, SO4, and HCO3 (mg/l) concentrations in surface water Mujib Dam Environ Monit Assess (2018) 190:604 Page 9 of 15 604 Fig. 9 Spatial distribution of NO3 and TN concentrations (mg/l) in surface water Mujib Dam 0.11 and 0.33 mg/l (averaging 0.2 mg/l) (Table 3). Water samples collected from the reservoir were below detection limits for F. Trace elements in water The statistics of trace metals in surface and near-bottom water samples are tabulated in Table 4. The majority of trace metal levels in reservoir water showed relatively similar surface and bottom values. The average surface concentrations of Cd, Cr, Cu, Fe, Mn, Pb, Zn, Co, Ni, and Sr were about 0.006, 0.014, 0.017, 0.209, 0.005, 0.011, 0.028, 0.040, 0.107, and 0.535 mg/l, respectively. Whereas, their average values in the lower water layer Table 3 Basic statistics of water quality data (mg/l) for surface samples collected from Mujib Dam (n = 20) NO2− NH4+ TN TP PO4−3 Min 0.004 0.005 1.73 0.11 0.02 Max 1.003 0.274 7.73 0.33 0.61 Mean 0.065 0.147 4.9 0.2 0.2 SD 0.221 0.084 1.38 0.07 0.13 were 0.018, 0.002, 0.013, 0.038, 0.010, 0.017, 0.044, 0.016, 0.063, and 0.445 mg/l, respectively. B was measured in the upper water and ranged between 0.1 and 0.31 mg/l with a mean of 0.172 mg/l. As described later, these low levels of trace elements suggest that they are probably of a geogenic origin (Al-Taani et al. 2013, 2014). Non-specific organic compounds and biological parameters The dissolved non-specific organic compounds (COD, BOD5) and biological parameters (chlorophyll a, E. coli, total coliform counts BTCC,^ thermotolerant coliform counts BTTCC^) are tabulated in Table 5. The concentrations of COD and BOD5 in surface water were relatively low. The COD levels varied from about 15 to 22.5 mg/l with an average value of 18.13 mg/l (Table 5). COD is likely to decrease in the following months due to the reduced algal biomass. The concentrations of BOD5 ranged between 2.2 and 6 mg/l with an average of about 3.2 mg/l. The spatial distribution patterns showed higher concentrations of BOD5 and COD that occurred in 604 Page 10 of 15 Environ Monit Assess (2018) 190:604 Table 4 Summary statistics of trace elements (mg/l) in water samples (surface and bottom) collected from Mujib Dam (n = 40 samples) Sample Cd Surface Bottom Cr Cu Fe Mn Pb Zn Co Ni Sr Min N.D N.D 0.007 0.022 N.D N.D N.D 0.011 N.D 0.296 Max 0.028 0.077 0.057 0.437 0.034 0.058 0.155 0.069 0.333 0.875 Mean 0.006 0.014 0.017 0.209 0.005 0.011 0.028 0.040 0.107 0.535 SD 0.010 0.019 0.010 0.114 0.009 0.016 0.038 0.020 0.111 0.175 Min N.D 0 0.003 N.D N.D N.D N.D 0.001 N.D 0.342 Max 0.074 0.010 0.042 0.083 0.078 0.062 0.167 0.039 0.765 0.721 Mean 0.018 0.002 0.013 0.038 0.010 0.017 0.044 0.016 0.063 0.445 SD 0.016 0.003 0.010 0.019 0.018 0.019 0.037 0.009 0.171 0.087 N.D not detected the northwestern corner, coincided with relatively higher levels of chlorophyll a (Fig. 10). While this suggests that COD and BOD5 in water are probably related to decomposition of dead algal mass, relatively weak correlation coefficients were found between BOD, COD, and chlorophyll a (r = 0.51 and 0.33, respectively). Agricultural runoff, improper waste disposal, and wastewater effluent discharged into the reservoir may contribute to the measured levels of COD and BOD5. Chlorophyll a concentrations in a lake are often used as an indicator of primary productivity. Chlorophyll a in surface water of Mujib Dam ranged between 1.12 and 3.7 μg/l (with an average of 2.21 μg/l). The average ratio of TN to TP in surface water is about 24.5, suggesting that phosphorus is the limiting factor for the growth of aquatic plants and algal blooms (Smith 1982). The relative high levels of nutrients necessary for eutrophication (0.2–0.3 mg/l for NO3 and 0.01 for PO4 (Lee and Lee 2005)) suggest phytoplankton blooms in Mujib Dam. In addition, based on the average concentrations of TN (4.9 mg/l) and TP Table 5 Summary statistics of COD, BOD5, chlorophyll a, E. coli, TCC, and TTCC in surface water samples collected from Mujib Dam (n = 20 samples) BOD5 mg/l COD Chl a μg/l E. coli* TCC* MPN/100 ml TTCC* Min 2.20 15 1.12 4 0 Max 6.00 22.50 3.70 26 1300 Mean 3.20 18.13 2.21 12.90 210.87 102.77 SD 0.91 2.03 0.83 5.26 354.41 243.53 1.80 920 *Data obtained from annual report of Royal Scientific Society 2014 (0.2 mg/l), the reservoir water is supposed to be in hypertrophic conditions (Håkanson and Jansson 1983). However, the average chlorophyll a level in surface water indicates oligo-mesotrophic water (Håkanson and Jansson 1983). In spite of the high nutrients availability (and high temperatures), algal growth is not stimulated, suggesting that other factors may have influenced algal growth. Salameh and Harahsheh (2011) observed extremely sparse phytoplankton populations in Mujib Dam and concluded that eutrophication blooms were limited in the Mujib Dam, probably because of high UV radiation. Water-rock interaction Gibbs (1970) plot was used to characterize the sources of dissolved chemical constituents in the reservoir water using ratio plots of Na+/(Na++Ca2+) vs. TDS (Fig. 11). The plot showed that all samples are located at the center indicating the dominance of weathering and dissolution of rocks. To better characterize the sources of ions in the reservoir water, especially if Ca and Mg were derived from dissolution of carbonate (calcite and dolomite) and evaporate (gypsum) minerals, the ionic ratios of (Ca+ Mg) to (SO4+HCO3) should be one (McLean and Jankowski 2000). The calculated ratio (meq/l) varied from 0.73 to 1.08 with a mean value of 0.92, with the majority of water samples showing a closer value to 1:1. This suggests that these ions were originated from dissolution of calcite, dolomite, and gypsum. In addition, the average molar ratios of Ca/HCO3 and Ca/Mg are 1.07 and 1.97, respectively, suggesting that calcite and probably dolomite are the major sources of these ions. Environ Monit Assess (2018) 190:604 Page 11 of 15 604 Fig. 10 Spatial distribution of BOD5, COD, and chlorophyll a concentrations (mg/l) in surface water Mujib Dam However, water samples showed above the 1:1 ration indicating an ion exchange process. Na:Cl ratio of one indicates that halite dissolution is responsible for Na and Cl (Fisher and Mullican 1997). The Na:Cl ratio ranged from 0.94 to 1.89, with most of the samples exhibiting molar ratio greater or equal to one, indicating that Na is not primarily associated with Cl (correlation coefficient is 0.31). It also suggests that Na may have been derived or increased due to ion exchange and halite dissolution (Cerling et al. 1989). K and Cl contents were weakly correlated (r = − 0.08) (Table 3). Similarly, negatively weak correlation was found between Na content in water and K (r = − 0.21) suggesting different sources for both ions. It is possible that they have partially been released from silicate-weathering and dissolution reactions (Meybeck 1987). Furthermore, the weak correlation between K and other major ions (Table 1) suggests that K was probably derived from K-feldspars (Batayneh et al. 2008; Al-Taani 2014) or fertilizers. The relative low K concentration is probably attributed to adsorption to clay minerals and/or formation of secondary minerals (Matthess 1982). TDS is correlated with Mg, Cl, SO4, HCO3, Ca, and K (Table 6). This suggests that weathering of carbonate and evaporate (gypsum) rocks is probably the major source of water chemistry (where these rocks cover a significant portion of Mujib watershed). A similar conclusion was reported by Jiries (2001). The increase in water use for irrigation, particularly during summer, would probably increase turbidity levels (re-suspension of settled particles) and subsequent dissolution of precipitated salts (as observed in Figs. 4 and 7). Classification of hydrochemical facies in the Mujib reservoir water using the Piper plot (Piper 1944) showed that the water is generally of a Ca-HCO3 type (Fig. 12). Table 6 Correlation coefficient matrices among water quality parameters of Mujib Dam TDS Fig. 11 Plot of TDS vs. Na/Na+Ca of water samples collected from Mujib Dam Ca Mg Ca 0.32 Mg 0.61 0.37 Na 0.24 0.50 0.22 K Na K Cl − 0.10 0.02 − 0.22 − 0.21 Cl− 0.80 0.33 0.69 0.31 − 0.08 SO4 0.80 0.35 0.71 0.32 − 0.16 0.87 HCO3 0.58 0.51 0.50 0.35 − 0.21 0.57 SO4 0.61 604 Page 12 of 15 Environ Monit Assess (2018) 190:604 Fig. 12 Piper diagram illustrating hydrochemical regime of Mujib water However, the water chemistry of Mujib Dam appeared to be governed by dissolution of carbonate and evaporate exposed in the watershed. Water chemistry showed the following ionic ratio: Ca > Na > Mg > K and HCO3 > SO4 > Cl. The water chemistry is classified as alkaline earth water with dominant bicarbonate, sulfate, and chloride. These results also suggest that the geogenic activities are primarily responsible for the occurrence of ions in waters (Batayneh and Al-Taani 2015). Positive correlation was observed between NO3 and TP (r = 0.89) (Table 3) suggesting that they are probably associated with agricultural runoff and wastewater discharged into the dam from Al-Lajoun wastewater treatment plant (Al-Malahmeh 2006; Manasreh et al. 2010). Phosphorus can also be leached from phosphate bearing strata in the catchment (Al-Shereideh et al. 2010) or released from the dam sediments through biological and chemical processes and mixed into the water column. The sediment particles that settled to the bottom of the lake may release phosphorus by re-suspension or low oxygen levels (Madison 1994; Yusuf et al. 2011). Continued high loading of sediments and nutrients to the reservoir will contribute to eutrophication process. Previous studies (Al-Malahmeh 2006; Manasreh et al. 2010) have reported relatively high concentrations of trace elements in Mujib water and sediments due to, among others, discharges of wastewater from Al-Lajoun wastewater treatment plant. However, our observation suggests that these low concentrations were primarily derived from natural sources and that the dam is less impacted by anthropogenic sources of pollution. The relative low levels of trace metals in Mujib water suggest that they have probably been released from sediments and/or leached from surrounding rocks and soils in the catchment during events of intense rainfall (occurs as flash flood). A plausible explanation for the low content of trace metals is that they may have been either precipitated (due to near-neutral pH and oxidizing conditions) or adsorbed onto metal oxides and clays. Metal release from sediment is affected by pH and salinity, where the lower pH and salinity, the higher the metals released (Gambrell et al. 1991; Lau and Chu 1999; Al-Rousan et al. 2016; Batayneh et al. 2015). Water quality for irrigation Water quality conditions in the reservoir are of particular concern because of increased water use for irrigation particularly in the Jordan Valley, where the common land use is agriculture. Except for Cd, TCC, and TTCC, all parameters tested showed values that are within the safe limits and concluded no restrictions on water use for irrigation purposes (Table 7). Environ Monit Assess (2018) 190:604 Page 13 of 15 604 Table 7 Results of average water analyses (surface, middle, and bottom waters) from Mujib reservoir collected in 2015, compared to Jordan guidelines for reclaimed domestic wastewater used for irrigation and Jordanian guidelines for drinking water Parameter Unit Mean n* JS 893/2006** JS 286/2008*** Mn mg/l 0.006 60 0.2 0.1 Ni 0.082 60 0.2 0.07 Cd 0.0141 60 0.01 0.003 Cu 0.015 60 0.2 1 Zn 0.04 60 5 4 Cr 0.007 60 0.1 0.05 Pb 0.013 60 0.2 0.01 Fe 0.099 60 5 1 Co 0.0244 60 0.05 – TDS 443.9 60 1500 1000 Ca 49.1 60 230 – Mg 15.4 60 100 – Na 35.3 60 230 200 K 7.3 60 – – Cl 39.7 60 400 500 NO3-N 0.294 60 30 11.3 (as NO3-N) TP 0.174 20 30 – TN 4.9 20 45 – SO4 83.2 60 500 500 HCO3 140.2 60 400 – F N.D 20 2 – TSS 5.6 20 50 BOD5 3.2 20 30 COD 18.13 20 100 DO 3.1 60 >2 TH 186.2 60 – 500 B 0.17 20 1 1 NH4 0.147 20 – 0.2 NO2 0.065 20 – 2 0 100 1.1 TCC MPN/100 ml 211+ TTCC MPN/100 ml 103+ 0 100 1.1 Temperature °C 20.4 60 – 25 Turbidity FTU 0.91 15 10 5 pH 7.4 60 6–9 6.5–8.5 SAR 1.10 60 9 – *Number of samples analyzed **Jordan guidelines for reclaimed domestic wastewater used for irrigation ***Jordanian guidelines for drinking water +Data obtained from the annual report of RSS 2014 N.D non-detected or below detection limit –Not required 604 Page 14 of 15 Water quality for drinking purposes In response to the growing concerns about the current shortages in drinking-water supply, the government has executed the Mujib desalination plant and conveyance project. The project aims to enhance the water supply of Karak region (southern Jordan) with 5 MCM annually of desalinated and treated water. It entails pumping of about 500 m2/h of desalinated and treated water from Mujib Dam to newly constructed reservoirs, which is then conveyed via an 18.5-km pipeline to Shihan and distributed to Karak region. Evaluation of water quality parameters for drinking use (based on Jordan standards of drinking water BJS 286/2008^) showed that Mujib reservoir waters are largely in accord with Jordan guidelines for drinking water except for Cd, TCC, TTCC, and E. coli (Table 7). This indicates that any water treatment plant installed should be capable of removing Cd (ultra-filtration system was installed in the water treatment unit). In addition, disinfection processes should be integrated to eradicate coliform bacteria and E. coli prior to use. For drinking water, BOD5 of less than 5 mg/l and COD of less than 10 mg/l are supposed to be acceptable. 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