Mixing Of Fresh Groundwater Research Articles

Assessment of hydrogeochemical status of groundwater in a coastal region of Southeast coast of India

A study was conducted in a coastal region of Cuddalore district of Tamil Nadu, India, to identify the hydrogeochemical processes controlling the groundwater chemistry. The major geological units of the study area are sandstone, clay, alluvium, and laterite soils of Tertiary and Quaternary age. A total of 64 groundwater samples were measured for major ions and stable isotopes. Higher electrical conductivity values indicate the poor quality groundwater along the coastal region. Saline water intrusion mainly affects the hydrochemical composition of the aquifer water reflected by Na–Cl-type waters. Cl−/(Cl− + HCO3−) ratio also indicates the mixing of fresh groundwater with saline water. The results of δD and δ18O analyses show that isotopic compositions of groundwater ranges from − 7.7 to − 2.1‰ for δ18O and from − 55.6 to − 18.5‰ for δD. Correlation and factor analysis were carried out to find the association of ions and to determine the major factors controlling the groundwater chemistry of the region. The study indicates that ion exchange, weathering, salt water intrusion along the coast, and anthropogenic impacts are the major controlling factors for the groundwater chemistry of the region.

Using hydrochemical and stable isotopic (δ2H, δ18O, δ11B, and δ37Cl) data to understand groundwater evolution in an unconsolidated aquifer system in the southern coastal area of Laizhou Bay, China

This study illustrates the origin of Quaternary brines and groundwater salinization processes at Laizhou Bay using hydrochemical and δ2H, δ18O, δ11B and δ37Cl data. The characteristics of the major ionic components of these brines demonstrated that brines originated from evaporated seawater. The δ2H and δ18O values of brines showed that seawater underwent strong evaporation and that these brines underwent mixing with paleo-meteoric water or surface water. The fact that the Cl/Br mass ratios of the Laizhou Bay brines (180–291) are lower than that of halite dissolution brine (>1000) contradicts the hypothesis that these brines were derived from halite dissolution, and the fact that the δ37Cl values (0.14‰–0.21‰) of these brines are more enriched than those derived from seawater evaporation represents positive evidence that evaporated seawater in the lagoon infiltrated through clay layer, during which ion filtration resulted in the positive Cl isotopic compositions of these brines. The higher B isotopic compositions (49.88‰–64.42‰) and lower B/Cl molar ratios (8.5 × 10–5∼1.6 × 10–4) of these brines are consistent with the brine origin determined based on the analysis of Cl isotopes. As the evaporated water infiltrated the clay layer, the preferential adsorption of 10B onto clay dominated the B isotopic composition of the brine. Five different mixing processes were calculated using B contents and B isotopic compositions. These calculations illustrated that one shallow saline groundwater (No. 05) was derived from the mixing of fresh groundwater and deep brine, thus indicating that brine intrusion occurred in the northern area of Changyi, which was caused by the over-exploitation of fresh groundwater. While one shallow saline groundwater (No. 10) and one deep saline groundwater (No. 12) in the northern of Xiaying were derived from the mixing of brine and seawater. The δ11B values of the two shallow brackish groundwater samples were close to that produced by the desorption of B from marine clay, and their B/Cl molar ratios were relatively higher, which demonstrated that B-desorption dominated the characteristic B hydrochemical compositions of brackish groundwater.

Minor element geochemistry of groundwater from an area with prevailing saline groundwater in Chikhwawa, lower Shire valley (Malawi)

Groundwater resources with high salinity content are found in some parts of the lower Shire River valley (Malawi). This paper discusses the geochemistry of minor elements with regards to the prevailing salinity. Hierarchical clustering and principal component analyses were used to identify factors which relate to both minor elements and samples and were interpreted as reflecting the influence of prevailing saline/brackish groundwater. Concentrations of lead (Pb), boron (B), strontium (Sr) and chromium (Cr) were associated with groundwater with high content of total dissolved solids (TDS). Speciation calculations indicated that dissolved Sr, barium (Ba) and lithium (Li) were mainly in the form of free aqueous ions whereas hydrolysed species were significant for aluminium (Al) and Cr, and carbonate complexes for Pb. Chloride complexes were prevalent for silver (Ag). Solubility of cerussite (PbCO3) and barite (BaSO4) was shown to act as a control on the levels of Pb and Ba, respectively. Thus, Ba concentrations were very low in saline groundwater owing to their high sulphate content. A relatively variable B concentration in the groundwater samples was explained using a binary mixing model of saline and fresh groundwater. The mixing of fresh groundwater with saline groundwater was concomitant with high Na+/Ca2+ ratios and enrichment of B, probably by desorption from clays. The WHO drinking water guidelines for Ba, B, Cr and Pb were exceeded in 6.5%, 9.7%, 16.1% and 64.5% of all the samples, respectively. However, all samples were below the Malawian specification of B in borehole and shallow well water quality (MS 733:2005) of 5.0mg/L.

Using 87Sr/86Sr, δ18O and δ2H isotopes along with major chemical composition to assess groundwater salinization in lower Shire valley, Malawi

Groundwater resources in some parts of the lower section of Shire River valley, Malawi, are not useable for rural domestic water supply due to high salinity. In this study, a combined assessment of isotopic (87Sr/86Sr, δ18O and δ2H) and major ion composition was conducted in order to identify the hydro-geochemical evolution of the groundwater and thereby the causes of salinity. Three major end-members (representing fresh- and saline groundwater, and evaporated recharge) were identified based on major ion and isotopic composition. The saline groundwater is inferred to result from dissolution of evaporitic salts (halite) and the fresh groundwater shows influence of silicate weathering. Conservative mixing models show that brackish groundwater samples result from a three component mixture comprising the identified end-members. Hence their salinity is interpreted to result from mixing of fresh groundwater with evaporated recharge and saline groundwater. On the other hand, the groundwater with low TDS, found at some distance from areas of high salinity, is influenced by mixing of evaporated recharge and fresh groundwater only. Close to the Shire marshes, where there is shallow groundwater, composition of stable isotopes of water indicates that evaporation may also be an important factor.

Ground Water Discharge of Agricultural Pesticides and Nutrients to Estuarine Surface Water

AbstractThis research investigated the transport of land‐applied nutrients and pesticides from unconfined aquifers to tidal surface waters of Virginia's coastal plain. Ground water, estuarine surface water, ground water discharge, upland soil, and offshore sediment samples were collected from May 1992 until February 1993 from four agricultural sites. Samples were analyzed for inorganic nitrogen and phosphorus and five pesticides: atrazine, cyanazine, alachlor, metolachlor, and carbofuran. Pesticides from aqueous samples were determined by liquid‐solid phase extraction followed by gas chromatography‐electron capture detection (GC‐ECD) and/or by pesticide‐specific immunoassay. Soil and sediment samples were analyzed by extraction and gas chromatography/mass spectrometry (GC/MS). Nutrient measurements indicated that fertilizer nitrogen was moving from the ground water to the surface water, and nitrogen fluxes across the sediment‐water interface were correlated with fresh water discharge rates. Mean nitrate‐N flux was 2.48 mg/m2hr, with a maximum value of 30.98 mg/m2hr. Pesticides were detected in more than half of the upland soil samples, in approximately 40 percent of the ground water samples, and in just under 20 percent of the seepage meter samples. Pesticides were not detected in any of the offshore sediment samples or surface water samples. Alachlor and metolachlor were detected in upland soil samples at concentrations ranging from 10 to almost 500 μg/kg. All five pesticides were found in ground water samples at concentrations generally below 1 μg/L, with alachlor, atrazine, and metolachlor most frequently found. Alachlor, atrazine, cyanazine, and metolachlor were detected in water discharging across the sediment‐water interface and entering estuarine waters at concentrations ranging from 0.05 to 0.5 μg/L. These levels were generally consistent with the amount of dilution due to the mixing of fresh ground water and saline pore waters prior to discharge across the sediment‐water interface. Based on all positive detections of pesticides in ground water discharge, which represented approximately 18 percent of all samples, average flux rates of cyanazine, metolachlor, alachlor, and atrazine were 0.32, 0.37, 0.80, and 1.12 μg/m2hr, respectively. These findings indicate that submarine ground water transport of both nutrients and pesticides does occur, and this transport route should be considered when implementing agricultural management practices. The levels of nitrogen transport to surface water appears significant. The overall levels of pesticide movement through ground water, although generally quite low, represent a transport route that is commonly neglected in watershed management.

Differential dissolution of a Pleistocene reef in the ground-water mixing zone of coastal Yucatan, Mexico

A geochemical explanation is provided for the extensive dissolution observed along the carbonate coast of the Yucatan Peninsula, Mexico. Mixing of fresh ground water with subterranean Caribbean seawater generates a highly reactive geochemical zone that enhances aragonite and calcite dissolution and permits neomorphism of aragonite. Mixing-zone dissolution caused by ground-water discharge is a major geomorphic process in developing caves, coves, and crescent-shaped beaches along the Yucatan coast. Such dissolution has probably been a significant control on permeability and porosity distribution in carbonate rocks in the geologic record.

Major element chemistry of the geothermal sea-water at Reykjanes and Svartsengi, Iceland

SummaryHigh-temperature geothermal fields in Iceland represent localized anomalies of hot, altered rock in the uppermost part of the crust, which coincide with points of maximum tectonic/magmatic activity. These points correspond to the intersection of oblique fault swarms to the plate boundaries. Geothermal activity under mid-ocean ridges follows probably similar tectonic/magmatic anomalies.Due to high permeability sea-water invades the bed-rock of the Reykjanes Peninsula, Iceland, and is overlain by a variably thick lens of dilute ground water of meteoric origin. The variable degree of salinity of geothermal waters in the Reykjanes Peninsula has resulted from different degree of mixing of fresh ground water with the underlying sea-water-ground-water in the downflow zones around the geothermal fields. At Reykjanes the geothermal water represents heated sea-water without any freshwater mixing. The difference in the composition of sea-water or sea-water/fresh water mixtures and the geothermal waters is due to basalt/water interaction at elevated temperatures. The major-element chemistry of the geothermal water represents an equilibrium composition at the relevant aquifer temperatures. The activities of silica, calcium, sulphate, and carbonate are thus limited by the solubilities of quartz, anhydrite, and calcite. Fluoride activity is thought to be controlled by an ionic exchange reaction where it substitutes for hydroxyl groups in phyllosilicates. The ratios of individual cations and hydrogen ion are governed by ionic exchange equilibria with hydrothermal minerals, probably smectite and chlorite. The equilibrium pH for the Reykjanes and Svartsengi geothermal waters is 5·5 and 5·1 respectively. Sea-water will become somewhat acid upon heating to more than about 300 °C and equilibration with basalt, the acidity increasing with temperature.