Isotope Hydrology

Groundwater from the La Paz arid coastal aquifer in Baja California, Mexico, is essentially the only source of drinking water for the local population and tourists, as well as irrigation water for agricultural needs. The intensive exploitation of the aquifer and water cycling has resulted in groundwater abatement (up to 10 m) and high salinity (up to *5800 mg l-1 ). A study using hydrochemistry, isotopic (deuterium, oxygen-18 and car- bon-14) and gaseous tracers (chlorofluorocarbons CFC-11, CFC-12, CFC-113), as well as multivariate statistics, was developed to elucidate groundwater composition, flow and occurrence. Groundwater is of meteoric origin, and a large proportion is subject to evaporation. The primary natural recharge is generated in the El Novillo and Las Cruces ranges, and groundwater subsequently flows in a SE–NW direction toward the coast. The initial water type is the result of discordant dissolution of silicate minerals and ion exchange on soils. In the lower plain portion, the aquifer system is recharged from irrigation return flow and sea- water intrusion, which significantly affects groundwater chemistry. Nitrate and chloride concentrations indicate that groundwater is highly affected by an overuse of fertilizers in agricultural activities, but there is little effect from urban activities. Seawater intrusion has progressed rapidly during the past decade, and the impact currently extends 13 km inland. Radiocarbon residence time calculations suggest that groundwater is modern, with the exception of Cha- metla and El Centerario sites in the central and western lowlands with ages of up to 5000 years. These waters indicate an additional recharge source for the upconing of fossil groundwater or regional flow.

The Chihuahua City region, located in the semiarid-arid northern Highlands of Mexico, has experienced intensive groundwater abstraction during the last 40 years to meet water demands in the region. A 15 geochemical survey was carried out to investigate the evolution from baseline to modern conditions of a 130-km flow path including the El Sauz-Chihuahua-Aldama-San Diego de Alcalá regions. The research approach included the use of major chemical elements, chlorofluorocarbons and environmental isotope ( 18 O, 2 H, 13 C and 14 C) tracers. Stable isotopes indicate that groundwater evolves from the evaporation of local rainfall and surface water. Groundwater located at the lower end of the flow section is up to 6,000 yr 20 old and older groundwater in the order of 9,000 yr BP was found in a deep well located in the upper part of the flow system, implying contribution from a neighbour basin. The background groundwater chemistry upstream of Chihuahua City, results from feldspar weathering. Beyond Chihuahua City the chemical conditions are strongly modified due to disposal of sewage from public and industrial water supply into the Rio Chuviscar, subsequent allocation of this water to agricultural irrigation areas and direct infiltration 25 under the river bed. In consequence, anions like chloride and sulphate are mainly related to surface sources.

The allocation of groundwater resources has been a challenge for many years due to its unforeseen side effect and lag time issues, which are often overlooked. The full impact of groundwater utilization/abstraction takes time to realize its effect at its full. In this paper, long‐term effects and groundwater dynamics were assessed using a water balance model and a time series analysis, respectively. Undeveloped groundwater systems are commonly found in a state of equilibrium, where, on average, equal amounts of water are recharged and discharged. A water budget is a static accounting of the state of the system at a given time, often before the system is developed. Water balance analysis was carried out together with the groundwater through flow, hydrograph, and surface‐groundwater interaction analysis (base flow index) to develop a conceptual water balance model, which is a very basic representation of a complex natural aquifer system and is instrumental to constrain and build a robust numerical model that can be readily justified and updated. A noble approach was employed to assess and constrain the discharge coming out of the model area to sustain the lake level, located to the north of the study area, using the whole lake catchment and lake water balance analysis. Based on the lake water balance, there is a deficit between input and output computation, and hence there should be a groundwater input to sustain the historical lake area. The analysis showed that the model area contributes 40% of the lake catchment, and hence the portion of the groundwater inflow feeding the lake was computed. This is one of the means to constrain the discharge, which adds more confidence to the recharge estimation. This is very important because the size of a sustainable groundwater development usually depends on how much of the discharge from the system can be captured by the development. Capture is independent of the recharge. Instead, it depends on the dynamic response of the aquifer system to the development. The idea that knowing the recharge is important in determining the size of a sustainable groundwater development is a myth and has no basis. The important entity in determining how a groundwater system reaches a new equilibrium is capture. How capture occurs in an aquifer system is a dynamic process. Following this study, lake water balance assessment was indirectly considered as prior information for the numerical model calibration of the discharge from the model area using a conductance parameter. Conductance is a key parameter to estimate the discharge volume together with the change in the simulated hydraulic head between time steps. The water balance error highlights which one is more sensitive, and this could help to assist in planning for future data collection/field work and where to invest the money. The water balance computation helps to figure out the degree of surface‐groundwater interaction, uncertainty, sensitive parameter, helps in the decision to invest time and money, and operates as a cross check with other analytical or numerical modelling.

With the increasing population, urbanization and industry in the arid area of Tecate, there is a concomitant increase in contaminants being introduced into the Tecate River and its aquifer. This contamination is damaging the usable groundwater supply and making local residents and commercial enterprises increasingly dependent on imported water from the Colorado River basin. In this study we apply a suite of chemical and isotopic tracers in order to evaluate groundwater flow and assess contamination trends. Groundwater recharge occurs through mountain-block and mountain-front recharge at higher elevations of the ranges. Groundwater from the unconfined, alluvial aquifer indicates recent recharge and little evolution. The increase in salinity along the flow
path is due to interaction with weathering rock-forming silicate minerals and anthropogenic sources such as urban wastewater, residual solids and agricultural runoff from fertilizers, livestock manure and/or septic tanks and latrines. A spatial analysis shows local differences and the impact of the infiltration of imported waters from the Colorado River basin. The general trend of impaired water quality has scarcely been documented in the last decades, but it is expected to continue. Since the
groundwater system is highly vulnerable, it is necessary to protect groundwater sources.

Groundwater chemistry and isotopic data from 40 production wells in the Atemajac and Toluquilla valleys, located in and around the Guadalajara metropolitan area, were determined to develop a conceptual model of groundwater flow processes and mixing. Stable water isotopes (δ2H, δ18O) were used to trace hydrological processes and tritium (3H) to evaluate the relative contribution of modern water in samples. Multivariate analysis including cluster analysis and principal component analysis were used to elucidate distribution patterns of constituents and factors controlling groundwater chemistry. Based on this analysis, groundwater was classified into four groups: cold groundwater, hydrothermal groundwater, polluted groundwater and mixed groundwater. Cold groundwater is characterized by low temperature, salinity, and Cl and Na concentrations and is predominantly of Na-HCO3-type. It originates as recharge at "La Primavera" caldera and is found predominantly in wells in the upper Atemajac Valley. Hydrothermal groundwater is characterized by high salinity, temperature, Cl, Na and HCO3, and the presence of minor elements such as Li, Mn and F. It is a mixed-HCO3 type found in wells from Toluquilla Valley and represents regional flow circulation through basaltic and andesitic rocks. Polluted groundwater is characterized by elevated nitrate and sulfate concentrations and is usually derived from urban water cycling and subordinately from agricultural return flow. Mixed groundwaters between cold and hydrothermal components are predominantly found in the lower Atemajac Valley. Twenty-seven groundwater samples contain at least a small fraction of modern water. The application of a multivariate mixing model allowed the mixing proportions of hydrothermal fluids, polluted waters and cold groundwater in sampled water to be evaluated. This study will help local water authorities to identify and dimension groundwater contamination, and act accordingly. It may be broadly applicable to other active volcanic systems on Earth.

Determining the isotopic composition of nitrate (NO3-) in water can prove useful to identify NO3- sources and to understand its dynamics in aquatic systems. Among the procedures available, the ‘ion-exchange resin method’ involves extracting NO3- from freshwater and converting it into solid silver nitrate (AgNO3), which is then analysed for 15N/14N and 18O/16O ratios. This study describes a simplified methodology where water was not pre-treated to remove Dissolved Organic Carbon (DOC) or barium cations (added to precipitate O-bearing contaminants), which suited samples with high NO3- (≥ 400 μM or 25 mg L-1 NO3-) and low DOC (typically < 417 μM of C or 5 mg L-1 C) levels. % N analysis revealed that a few AgNO3 samples were of low purity (compared with expected % N of 8.2), highlighting the necessity to introduce quality control/quality assurance procedures for silver nitrate prepared from field water samples. Recommendations were then made to monitor % N together with % O (expected at 28.6, i.e. 3.5 fold % N) in AgNO3 in order to better assess the type and gravity of the contamination as well as to identify and reject unreliable data.

Recent studies using water-stable isotopes (δ18O and δ2H) have suggested an ecohydrological separation of water flowing to streams or recharging groundwater and water used by trees, known as the 'two water worlds' (TWW) hypothesis. In this study, we measured water isotopic composition in precipitation [open field and throughfall, i.e. local meteoric water line (LMWL)] and the mobile water compartment (i.e. stream and soil solution), bulk soil water and xylem water over a period of 1.5 years in two headwater catchments: NF, covered with old growth native evergreen forest (Aetoxicon punctatum, Laureliopsis philippiana and Eucriphya cordifolia), and EP, covered with 4 and 16-year-old Eucalyptus nitens stands. Our results show that precipitation, stream and soil solution plot approximately along the LMWL, while xylem waters from all studied tree species plot below the LMWL, supporting the TWW hypothesis. However, we also found evidence of ecohydrological connectivity during the wet season, likely controlled by the amount of antecedent precipitation. These observations hold for all investigated tree species. On both sites, a different precipitation source for stream and xylem water was observed. However, in EP, bulk soil showed a similar precipitation source as xylem water from both E. nitens stands. This suggests that E. nitens may use water that is recharging the bulk soil compartment. We conclude that under a rainy temperate climate, the TWW hypothesis is temporal and does not apply during wet seasons.

Radioactive isotope tritium (3H) and stable isotopes of hydrogen (2H/1H) and oxygen (18O/16O) were measured during 2010 in the Sava River, precipitation and groundwater at 3 monitoring wells and 1 production well of the Petruševec aquifer, close to the Sava River in the Zagreb area. Significant increase of 3H activity in the Sava River was observed in June, (200 ± 20) TU, and in groundwater of all wells with damped response (maximum 60 TU) and with delay of 3–5 months related to the Sava River. This increase was explained by release of tritiated water from the Krško Nuclear Power Plant, 30 km upstream from Zagreb in the beginning of June 2010. Stable isotope analyses showed similar range of δ2H and δ18O values for the Sava River and groundwater samples with higher variations in surface water. Differences in monthly variations of δ18O values between particular monitoring wells, together with 3H values, indicated different infiltration times of surface water of the Sava River to different wells of the Petruševec aquifer