RESUMEN El objetivo de esta investigación fue evaluar los niveles de concentración de nitratos y sulfatos en las aguas de escorrentía superficial de las microcuencas Huangamarca y Pollo de la provincia Otuzco, región La Libertad. Los estudios experimentales de las aguas captadas de las escorrentías superficiales en los agroecosistemas permitieron determinar concentraciones promedio de nitratos de 0,214 meq/l y 0.662 meq/l en las microcuencas Huangamarca y Pollo, respectivamente. Los sulfatos fluctuaron entre 0,435 meq/l en Huangamarca y 1,321 meq/l en Pollo. Estos valores de acuerdo a los niveles permisibles están por debajo de los niveles de contaminación, según los estándares de calidad de agua para riego. Se concluye que las aguas de las microcuencas estudiadas en Huangamarca y Pollo no tienen riesgos para su utilización. | ABSTRACT The objective of this research was to evaluate the concentration levels of nitrates and sulfates in the surface runoff waters of the Huangamarca and Pollo micro-watersheds in Otuzco Pro^vince, La Libertad Region. Experimental studies of the water collected from surface runoff in the agroecosystems determined average nitrate concentrations of 0.214 meq/l and 0.662 meq/l in the Huangamarca and Pollo micro-watersheds, respectively. Sulfates ranged from 0.435 meq/l in Huangamarca to 1.321 meq/l in the Pollo microbasin. These values, according to the permissible levels, are below the contamination levels according to the water quality standards for irrigation. It is concluded that the waters of the Huangamarca and Pollo micro-watersheds studied are safe for use.

This work focuses on the geochemical processes taking place in the acid drainage in the Ribeira da Água Forte, located in the Aljustrel mining area in the Iberian Pyrite Belt. The approach involved water and stream sediment geochemical analyses, as well as other techniques such as sequential extraction, Mössbauer spectroscopy, and X-ray diffraction. Ribeira da Água Forte is a stream that drains the area of the old mine dumps of the Aljustrel mine, which have for decades been a source of acid waters. This stream flows to the north for a little over than 10km, but mixes with a reduced, organic-rich, high pH waste water from the municipal waste water pools of the village. This water input produces two different results in the chemistry of the stream depending upon the season: (i) in the winter season, effective water mixing takes place, and the flux of acid water from the mine dumps is continuous, resulting in the immediate precipitation of the Fe from the acid waters; (ii) during the summer season, acid drainage is interrupted and only the waste water feeds the stream, resulting in the reductive dissolution of Fe hydroxides and hydroxysulfates in the stream sediments, releasing significant quantities of metals into solution. Throughout the year, water pH stays invariably within 4.0–4.5 for several meters downstream of this mixing zone even when the source waters come from the waste water pools, which have a pH around 8.4. The coupled interplay of dissolution and precipitation of the secondary minerals (hydroxides and sulfates), keeps the system pH between 3.9 and 4.5 all along the stream. In particular, evidence suggests that schwertmannite may be precipitating and later decomposing into Fe hydroxides to sustain the stream water pH at those levels. While Fe content decreases by 50% from solution, the most important trace metals are only slightly attenuated before the solution mixes with the Ribeira do Rôxo stream waters. Concentrations of As are the only ones effectively reduced along the flow path. Partitioning of Cu, Zn and Pb in the contaminated sediments also showed different behavior. Specific/non-specific adsorption is relevant for Cu and Zn in the upstream branch of Ribeira da Água Forte with acid drainage conditions, whereas the mixture with the waste water causes that the association of these metals with oxyhydroxide to be more important. Metals bound to oxyhydroxides are on the order of 60–70% for Pb, 50% for Cu and 30–60% for Zn. Organic matter is only marginally important around the waste water input area showing 2–8% Cu bound to this phase. These results also show that, although the mixing process of both acid and organic-rich waters can suppress and briefly mitigate some adverse effects of acid drainage, the continuing discharge of these waste waters into a dry stream promotes the remobilization of metals fixed in the secondary solid phases in the stream bed back into solution, a situation that can hardly be amended back to its original state.

With depleted coal resources or deteriorating mining geological conditions, some coal mines have been abandoned in the Fengfeng mining district, China. Water that accumulates in an abandoned underground mine (goaf water) may be a hazard to neighboring mines and impact the groundwater environment. Groundwater samples at three abandoned mines (Yi, Er and Quantou mines) in the Fengfeng mining district and the underlying Ordovician limestone aquifer were collected to characterize their chemical and isotopic compositions and identify the sources of the mine water. The water was HCO₃·SO₄-Ca·Mg type in Er mine and the auxiliary shaft of Yi mine, and HCO₃·SO₄-Na type in the main shaft of Quantou mine. The isotopic compositions (δD and δ¹⁸O) of water in the three abandoned mines were close to that of Ordovician limestone groundwater. Faults in the abandoned mines were developmental, possibly facilitating inflows of groundwater from the underlying Ordovician limestone aquifers into the coal mines. Although the Sr²⁺ concentrations differed considerably, the ratios of Sr²⁺/Ca²⁺ and ⁸⁷Sr/⁸⁶Sr and the ³⁴S content of SO₄²⁻ were similar for all three mine waters and Ordovician limestone groundwater, indicating that a close hydraulic connection may exist. Geochemical and isotopic indicators suggest that (1) the mine waters may originate mainly from the Ordovician limestone groundwater inflows, and (2) the upward hydraulic gradient in the limestone aquifer may prevent its contamination by the overlying abandoned mine water. The results of this study could be useful for water resources management in this area and other similar mining areas.

Goodenough Spring (Texas, USA) is a large spring near the border of the American state of Texas and the Mexican state of Coahuila, discharging into the international Amistad Reservoir on the river Rio Grande (Rio Bravo). Discharge was routinely measured from 1928 until 1968 to partition the flow of the river between the two countries in accordance with water-use treaties. Samples were analyzed for water-quality parameters in 1967–1968 prior to inundation under 45 m of Amistad Reservoir in 1968. Subsequently, discharge has been estimated indirectly by the International Boundary and Water Commission (IBWC). For the first direct measurements of the spring in 37 years, velocity and cross-sectional measurements were made and water samples collected in the summer of 2005 using advanced self-contained underwater breathing apparatus (SCUBA) techniques. Spring discharge was calculated at 2.03 m³ s–¹, approximately one-half of the historical mean of 3.94 m³ s–¹. In situ and laboratory analyses of samples for temperature, pH, dissolved oxygen, specific conductance, alkalinity, nitrate-nitrogen, dissolved solids, chloride, sulfate, fluoride, phosphorus, calcium, sodium, potassium, magnesium, and iron showed the water quality to be very good for human consumption and crop irrigation. Measurement values are relatively unchanged from those reported 37 years prior.

Groundwater flow and the geochemical evolution of a freshwater lens in an aquifer on a Pacific atoll were investigated by hydrogeochemical surveys. Sulfur hexafluoride measurements showed that deeper groundwater and groundwater at the periphery of the lens are older, consistent with a downward and outward groundwater flow scheme. This is the typical flow scheme on Pacific atolls where a Holocene–Pleistocene unconformity restricts the shape of the freshwater lens. Enrichment of Mg/Ca in the groundwater is another indicator of a longer residence time, because contact between the groundwater and the carbonate sediments composing the aquifer leads to the release of Mg from high-Mg calcite and the precipitation of Ca as low-Mg calcite. Groundwater quality was also affected by anthropogenic nitrogen loading and aboveground organic matter, which were altered by denitrification and sulfate reduction in the aquifer, especially in the older groundwater. The chemical composition of the groundwater in the center of the island, where saline water is up-coning, implies that freshwater recharge dilutes the older saline water, which as time passes will eventually be replaced by newly recharged freshwater.

An approach is presented to investigate the regional evolution of groundwater in the basin of the Amacuzac River in Central Mexico. The approach is based on groundwater flow cross-sectional modeling in combination with major ion chemistry and geochemical modeling, complemented with principal component and cluster analyses. The hydrogeologic units composing the basin, which combine aquifers and aquitards both in granular, fractured and karstic rocks, were represented in sections parallel to the regional groundwater flow. Steady-state cross-section numerical simulations aided in the conceptualization of the groundwater flow system through the basin and permitted estimation of bulk hydraulic conductivity values, recharge rates and residence times. Forty-five water locations (springs, groundwater wells and rivers) were sampled throughout the basin for chemical analysis of major ions. The modeled gravity-driven groundwater flow system satisfactorily reproduced field observations, whereas the main geochemical processes of groundwater in the basin are associated to the order and reactions in which the igneous and sedimentary rocks are encountered along the groundwater flow. Recharge water in the volcanic and volcano-sedimentary aquifers increases the concentration of HCO₃ –, Mg²⁺ and Ca²⁺ from dissolution of plagioclase and olivine. Deeper groundwater flow encounters carbonate rocks, under closed CO₂ conditions, and dissolves calcite and dolomite. When groundwater encounters gypsum lenses in the shallow Balsas Group or the deeper Huitzuco anhydrite, gypsum dissolution produces proportional increased concentration of Ca²⁺ and SO₄ ²–; two samples reflected the influence of hydrothermal fluids and probably halite dissolution. These geochemical trends are consistent with the principal component and cluster analyses.

Major ion geochemistry reveals that the hydrochemical evolutionary process of karst groundwater in the northeastern Huaibei Plain, China, consists of three sub-processes: the dissolution of dolomite, gypsum dissolution with dedolomitization, and mixing with overlying pore water. Understanding hydrochemical evolution has been an important topic in understanding the history, status, and dynamics of the groundwater flow system. The presented study found a hydrochemical boundary roughly corresponding to the thickness of overlying strata equating to 50 m depth, indicating two flow compartments participating in different hydrological cycles—a local shallow rapidly replenished compartment showing lower and more stable main ion concentrations, and a regional deep-flow compartment showing higher and sporadic concentrations of Na⁺, K⁺, Ca²⁺, Mg²⁺, Cl⁻ and SO₄²⁻, as well as high total dissolved solids (TDS), total hardness, and sodium adsorption ratio (SAR). In areas with aquifers with low water transmitting ability, groundwater samples show a high chloride ratio and elevated TDS values, indicating salinization of groundwater due to stagnant water flows. Analyses of the data on the saturation indexes and mineral solutions, in tandem with trilinear diagram analysis and petrological observations, indicate that dedolomitization is the dominant process controlling the chemical characteristics of karst groundwater in the study area. Groundwater and pore-water mixing was also observed at the later evolutionary stage of groundwater flow, demonstrating frequent groundwater/pore-water interactions where groundwater is recharged by pore water due to lower groundwater level in the study area.

Groundwater is typically the only water source in arid regions, and its circulation processes should be better understood for rational resource exploitation. Stable isotopes and major ions were investigated in the northeastern Tengger Desert, northern China, to gain insights into groundwater recharge and evolution. In the northern mountains, Quaternary unconsolidated sediments, exposed only in valleys between hills, form the main aquifer, which is mainly made of aeolian sand and gravel. Most of the mountain groundwater samples plot along the local meteoric water line (LMWL), with a more depleted signature compared to summer precipitation, suggesting that mountain groundwater was recharged by local precipitation during winter. Most of the groundwater was fresh, with total dissolved solids less than 1 g/L; dominant ions are Na⁺, SO₄ ²⁻ and Cl⁻, and all mineral saturation indices are less than zero. Evaporation, dissolution and cation exchange are the major hydrogeochemical processes. In the southern plains, however, the main aquifers are sandstone. The linear regression line of δD and δ ¹⁸O of groundwater parallels the LMWL but the intercept is lower, indicating that groundwater in the plains has been recharged by ancient precipitation rather than modern. Both calcite and dolomite phases in the plains groundwater are close to saturation, while gypsum and halite can still be dissolved into the groundwater. Different recharge mechanisms occur in the northern mountains and the southern plains, and the hydraulic connection between them is weak. Because of the limited recharge, groundwater exploitation should be limited as much as possible.

Enhanced bioremediation combined with in-situ chemical oxidation has the potential to remediate groundwater contaminated with organics. To explore the remediating effects of these two approaches and to evaluate their combined feasibility, traditional gasoline (no ethanol) and ethanol-gasoline (10% ethanol, v/v) were released into experimental sand tanks (TG-tank and EG-tank, respectively) under the same water-flow conditions. Nitrate and sulfate were added to enhance bioremediation and then persulfate was injected to encourage chemical oxidation. Two push–pull tests, using persulfate and bromide respectively, were conducted to compare their behavior. The results showed that nitrate reduction, rather than sulfate reduction, enhanced BTEX (benzene, toluene, ethylbenzene, and xylene) biodegradation, but the presence of ethanol inhibited these processes. The detected concentration of BTEX in the TG-tank was lower than that in the EG-tank, and the first-order decay rate constants of BTEX in the TG-tank and EG-tank under nitrate-adjusted conditions were 0.0058 and 0.0016 d⁻¹, respectively. The first persulfate injection (10 g L⁻¹) resulted in 86 and 94% concentration decreases of BTEX in the TG-tank and EG-tank, respectively, at first-order decay rates of 0.0180 and 0.0181 d⁻¹, respectively. However, the subsequent persulfate injections at 20 and 50 g L⁻¹ had no significant removal effect on BTEX. Persulfate oxidation made pH decrease (but it quickly recovered) and did not significantly inhibit nitrate reduction. This study suggests that enhanced nitrate reduction can be combined with persulfate oxidation for the in-situ remediation of groundwater contaminated by petroleum hydrocarbons.

Groundwater recharge and evolution in the Shule River basin, Northwest China, was investigated by a combination of hydrogeochemical tracers, stable isotopes, and radiocarbon methods. Results showed the general chemistry of the groundwater is of SO₄ ²⁻ type. Water–rock reactions of halite, Glauber’s salt, gypsum and celestite, and reverse ionic exchange dictated the groundwater chemistry evolution, increasing concentrations of Cl⁻, Na⁺, SO₄ ²⁻, Ca²⁺, Mg²⁺ and Sr²⁺ in the groundwater. The δ¹⁸O and δ²H values of groundwater ranged from −10.8 to −7.7 and −74.4 to −53.1 ‰, respectively. Modern groundwater was identified in the proluvial fan and the shallow aquifer of the fine soil plain, likely as a result of direct infiltration of rivers and irrigation returns. Deep groundwater was depleted in heavy isotopes with ¹⁴C ages ranging from 3,000 to 26,000 years, suggesting palaeowater that was recharged during the late Pleistocene and middle Holocene epochs under a cold climate. These results have important implications for groundwater management in the Shule River basin, since large amounts of groundwater are effectively being mined and a water-use strategy is urgently needed.