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<jats:title>Abstract</jats:title><jats:p>The performance of time series models is assessed using synthetic head series simulated with a numerical model that solves Richards' equation for variably saturated flow. Heads were simulated in a homogeneous unconfined aquifer between two parallel canals; measured daily precipitation and potential evaporation are specified at the land surface and root water uptake is simulated. The head response to a precipitation event is nonlinear and depends on the saturation degree and rainfall before and after the precipitation event while evaporation reduction occurs during summers. Synthetic series were generated for 27 years and three different soil types; the unsaturated zone thickness varies between 0 and &gt;5 m. The synthetic head series were simulated with a linear and nonlinear time series model. Performance of a linear time series model with four parameters, using a scaled Gamma response, gave <jats:italic>R</jats:italic><jats:sup>2</jats:sup> values ranging from 0.67 to 0.96. The nonlinear time series model with five parameters simulates recharge using a root zone reservoir after which the head response to recharge is simulated with a scaled Gamma response function. The nonlinear time series model was able to simulate all synthetic head series very well with <jats:italic>R</jats:italic><jats:sup>2</jats:sup> values above 0.9 for almost all models. The head response of the nonlinear model to a single precipitation event compares well to the response of the variably saturated groundwater model. The provided scripts may be used to simulate synthetic head series for other climates or for systems with additional complexity to assess the performance of other data‐driven models.</jats:p>

<jats:title>Abstract</jats:title><jats:p>Attributing the sources of legacy contamination, including brines, is important to determine remediation options and to allocate responsibility. To make sound remediation decisions, it is necessary to distinguish subsurface sources, such as leaking oil and gas (“O&amp;G”) wells or natural upward fluid migrations, from surface releases. While chemical signatures of surface and subsurface releases may be similar, they are expected to imprint specific dissolved noble gas signatures, caused by the accumulation of terrigenic noble gases in subsurface leaks or re‐equilibration of noble gases following surface releases. We demonstrate that only a historic surface release influenced the dissolved noble gas signature of groundwater in monitoring wells contaminated with brine near an abandoned O&amp;G well, rather than subsurface leakage from the well. Elevated brine concentrations were associated with lower terrigenic helium concentrations, indicating re‐equilibration with atmospheric helium at the surface during the release. Geophysical surveying indicating elevated salinity in surficial soils upgradient of the wells further supported the interpretation of the noble gas data. Eliminating the possibility that subsurface leakage was the source of the plume was critical to selecting the proper remedial action at the site, which otherwise may have included an unnecessary and costly well re‐abandonment. This study demonstrates the use of noble gas analysis to compare potential sources of brine contamination in groundwater and to exclude subsurface leakage as a potential source in an oilfield.</jats:p>

<jats:title>Abstract</jats:title><jats:p>The Arab region is located in an arid environment and suffers from water scarcity and poor water quality which are expected to become more severe in coming years due to global warming. In this study, the groundwater quality of 205 wells in Qatar was investigated. The physical parameters of pH, electrical conductivity (EC), total dissolved solids (TDS), salinity, inorganic carbon (IC), and organic carbon (OC) were determined. The study characterized the concentrations of major anions of Cl, F, Br, NO<jats:sub>3</jats:sub>, PO<jats:sub>4</jats:sub>, and SO<jats:sub>4</jats:sub>, and major cations of Ca, K, Mg, and Na. Importantly, metals and metalloids including V, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Se, Mo, Cd, Ba, Pb, and U were determined. The results revealed that the groundwater of all wells is not drinkable due to high salinity (average TDS 4598 mg/L and salinity 0.4%, respectively). Additionally, average concentrations of major anions Cl, SO<jats:sub>4</jats:sub>, and F were 1472, 1064, and 1.9 mg/L, respectively, and all exceed the World Health Organization (WHO) guidelines for drinking water. However, NO<jats:sub>3</jats:sub> concentration in 11 out of 205 wells was above the WHO guidelines of 50 mg/L due to intensive agriculture and fertilizer applications. Major cations of Ca, K, Mg, and Na were higher than WHO guidelines with average concentrations of 345, 63, 127, and 923 mg/L, respectively. All trace metals were much lower than the WHO guidelines for drinking water; however, the vanadium (V) average concentration in groundwater of all wells was 31 μg/L, which is five times higher than the Dutch guidelines (whereas the WHO has no guidelines for V). The groundwater of Qatar is dominated by Ca and Mg sulfates in Sabkha environments and dominated by NaCl in the coastal zones from evaporate environments consisting of coastal salt flats, salt pans, estuaries, and lagoons supersaturated by salts and the influence of sea water intrusion.</jats:p>

<jats:title>Abstract</jats:title><jats:p>Water constitutes an indispensable resource vital for sustaining life. In this context, groundwater stands out as a paramount global water source. Throughout history, underground dams (UGDs) have been employed to augment the storage capacity of local aquifers. This study employs a multistep elimination approach to identify optimal locations for constructing UGDs in the Bursa district, Turkey. Initially, the Digital Elevation Model (DEM) is utilized to pinpoint the potential construction sites at the watershed scale. Criteria such as suitable topographic slope range, proximity to the transport infrastructures, presence of natural or artificial reservoirs, distance to active or inactive faults, proximity to the urban and rural settlements, location of the irrigation zones, geological conditions, distance to the consumption hubs, thickness of alluvium layer, and the groundwater depth are used to establish the buffer zones for exclusion of potential sites. Then, storage volume in the proposed sites is determined, and formal requests from the local communities are taken into consideration for determining the best UGD sites. The study concludes that five UGDs for irrigation and one for drinking water purposes could be recommended for further implementation.</jats:p>

<jats:title>Abstract</jats:title><jats:p>Open pit mining frequently requires regional water tables to be lowered to access ore deposits. When mines close, dewatering ceases allowing the water table to recover. In arid and semi‐arid mining regions, the developing pit lakes are predominantly fed by groundwater during this recovery phase and pit lakes develop first into “terminal sinks” for the surrounding groundwater system. With time, the re‐establishment of regional hydraulic gradients can cause pit lakes to develop into throughflow systems, in which pit lake water outflows into adjacent aquifers. In this study, we use numerical groundwater modeling to aid process understanding of how regional hydraulic gradients, aquifer properties, net evaporation rates, and pit geometry determine the hydraulic evolution of groundwater‐fed pit lakes. We find that before the recovery of the regional water table to its new equilibrium, pit lakes frequently transition to throughflow systems. Throughflow from pit lakes to downstream aquifers can develop within two decades following cessation of dewatering even under low hydraulic gradients (e.g., 5 × 10<jats:sup>−4</jats:sup>) or high net evaporation rates (e.g., 2.5 m/year). Pit lakes remain terminal sinks only under suitable combinations of high evaporation rates, low hydraulic gradients, and low hydraulic conductivities. In addition, we develop an approximate analytical solution for a rapid assessment of the hydraulic status of pit lakes under steady‐state conditions. Understanding whether pit lakes remain terminal sinks or transition into throughflow systems largely determines the long‐term water quality of pit lakes and downstream aquifers. This knowledge is fundamental for mine closure and planning post‐mining land use.</jats:p>