Variations In Groundwater Chemistry Research Articles

Dynamical variations in groundwater chemistry influenced by intermittent water delivery at the lower reaches of the Tarim River

The water of Bosten Lake was released to lower reaches of the Tarim River for 5 times from 2000 to 2002. The changes of total dissolved solid (TDS) and the major ions (SO42−, Cl−, Na+, Ca2+, Mg2+ and HCO3−) were analyzed during this period. It was found out that TDS and the concentrations of the major ions initially and quickly increased and then decreased, but finally increased again. These changes were different at different distances from the river, which indicated that the groundwater changes relied on the distance from the river. In addition, the salt in groundwater was only diluted but not removed by the water. It was suggested that ecological measures should be sought to really promote the quality of the groundwater at the lower reaches of the Tarim River.

The water quality of streams draining a plantation forest on gley soils: the Nant Tanllwyth, Plynlimon mid-Wales

Abstract. The water quality of the Nant Tanllwyth stream in the Plynlimon region of mid-Wales is related to the key hydrobiogeological controls and the effects of conifer harvesting based on an analysis of rain, cloud, stream and groundwater measurements. The results show the normal patterns of stream water quality response to hydrology. Thus, there is a high damping of atmospheric inputs due to storage in a highly heterogeneous soil and groundwater system. Correspondingly, there is a highly dynamic response for components such as calcium, bicarbonate and aluminium. This response links to the relative inputs of acidic and aluminium-bearing soil waters under high flow conditions and base enriched bicarbonate bearing waters from the groundwater areas under baseflow conditions. The introduction of a deep borehole near the main stem of the river opened up a groundwater flow route to the stream and other parts of the catchment. There were two aspects to this. Firstly, it caused a change to the stream water quality, particularly under baseflow conditions, by increasing the concentrations of calcium and magnesium and by reducing the acidity. The monitoring shows that this change has persisted for over eight years and that there is no sign of reversion to pre-borehole times. Secondly, it caused a change in the groundwater level and chemistry at a borehole on the other side of the river. This feature shows that the fracture system is of hydrogeochemical and hydrogeological complexity. The effects of conifer harvesting are remarkable. At the local scale, felling leads to the expected short term increase in nitrate, ammonium and phosphate from the disturbance of the soil and the reduction in uptake into the vegetation. Correspondingly, there is a reduction in sodium and chloride linked to reduced scavenging of atmospheric inputs from cloud water by the vegetation and also due to increased dilution potential due to reductions in transpiration by the trees. However, within the main stream, virtually no change is observed in stream water quality with felling, except for a decrease rather than an expected increase in nitrate concentration. It seems that the increase in phosphate, for a system that is essentially phosphate limiting, has stimulated biological activity in the stream leading to increased uptake of nitrate and ammonium. However, there is little change in the sodium and chloride in the stream and there are important issues of the representative nature of studying small scale drainage areas. Boreholes, introduced to assess the impacts of the felling programme, show unexpected variations in groundwater chemistry. These variations are associated with the complexity of both flow routing and the chemical reactivity within the groundwater and lower soil zones, rather than changes that can be linked specifically to felling. The implications of the study are discussed in relation to both process understanding and forestry management practices. Keywords: water quality, acidification, conifer, harvesting, pH, nitrate, ammonium, phosphate, nutrients, alkalinity, Plynlimon, Wales, Tanllwyth, streams, groundwater

Natural baseline quality of Madrid Tertiary Detrital Aquifer groundwater (Spain): a basis for aquifer management

The Madrid Tertiary Detrital Aquifer is one of the largest and most important aquifers of Spain. This paper assesses the most relevant controls on the natural baseline quality and the dominant chemical processes within the aquifer. The hydrochemistry of the groundwater is variable despite the relative uniformity of the detrital sediments. The natural baseline is expressed as a range of values that are controlled by lithological and hydrological factors; spatial variations of groundwater chemistry are related to changes in rock type, water-rock interaction and the residence time of groundwater. The fundamental chemical processes within the Arkosic aquifer are hydrolysis of silicates, dissolution of carbonates, dissolution of evaporites (only in the vicinity of the transitional facies), ion exchange, neoformation of clays, precipitation of silica as cement, and precipitation of carbonates due to increasing temperature along the downward pathways. Some chemical and physico-chemical parameters like pH, dissolved oxygen, and hardness, and several elements like calcium, sodium, magnesium, silica, and arsenic show an evolutionary trend according to groundwater flow path. A gradual increase in arsenic concentration from recharge areas to discharge areas is observed; it is the main natural water constituent that deteriorates the quality of the fresh Madrid groundwater as a drinking water supply. The occasionally elevated arsenic concentrations originate from natural sources. The concentration and mobility of arsenic seems to be controlled by pH-dependent anion exchange processes resulting from the evolution to Na-HCO3 water.

Hydrogeochemistry of groundwater in coastal wetlands: implications for coastal conservation in Scotland

Groundwater in a shallow coastal aquifer in north east Scotland was monitored over the hydrological year October 1996–September 1997. Groundwater flow from inland areas sustained freshwater conditions in a dune–wetland complex of conservation importance. In particular, seasonal flooding of the coastal wetlands due to water table rise provided important roosting and breeding habitats for waterfowl. Hydrogeochemical analysis revealed that groundwater in the shallow sand aquifer was circum-neutral, and non-saline, despite being within 50 m of the sea and only 1 m above the mean high water mark. Calcium and HCO3− were the dominant cation and anion respectively, reflecting weathering processes in the aquifer. Use of the geochemical code NETPATH indicated that calcite weathering in shell fragments within the sand was the primary source of Ca and alkalinity generation. The concentrations of Na and Cl were also important, though these can be explained primarily by atmospheric inputs from precipitation. In detail, the spatial and temporal variation in groundwater chemistry was remarkably complex for what intuitively appeared a simple aquifer system. Temporal variations in groundwater chemistry mainly related to the seasonal event of groundwater recharge. Thus, the main period of rising groundwater levels resulted in a marked dilution of solutes in the aquifer, implying that water storage greatly increased in a relatively short period. A period of several weeks appeared to be required for dissolution processes to proceed to equilibrium. Spatial variation in groundwater chemistry appears to relate to the spatial distribution of geochemical processes in different hydrogeological units. Sulphate reduction, alkalinity generation and Fe precipitation appear to be locally important processes. The chemistry of groundwater maintains the wetland habitat by providing freshwater conditions that allow populations of various plant species to flourish. The potentially large recharge catchments of coastal wetlands, together with increasing pressures in the coastal zone, dictate that pollution can threaten the integrity of hydrochemical processes and requires careful monitoring if freshwater wetlands are to maintain their conservation importance.

The Distribution of Microbial Communities in Anaerobic and Aerobic Zones of a Shallow Coastal Plain Aquifer.

Randomly amplified polymorphic DNA (RAPD) fingerprinting was used to determine the genetic similarity of whole-community DNA extracts from unattached microorganisms in several groundwater wells. The study site was a shallow coastal plain aquifer on the Eastern Shore of Virginia that contains distinct regions of anaerobic and aerobic groundwater. Several wells in each region were sampled, and principal component and cluster analyses showed a clear separation of the microbial communities from the two chemical zones of the aquifer. Within these zones, there was no relationship between the genetic relatedness of a pair of communities and their spatial separation. Two additional sets of samples were taken at later times, and the same clear separation between communities in the different zones of the aquifer was observed. The specific relationships between wells within each zone changed over time, however, and the magnitude and direction of these changes corresponded to concurrent changes in the groundwater chemistry at each well. Together, these results suggest that local variation in groundwater chemistry can support genetically distinct microbial communities, and that the composition of the microbial communities can follow seasonal fluctuations in groundwater chemistry.

Hydrogeochemistry of shallow groundwater in an upland Scottish catchment

The hydrogeochemistry of shallow groundwater has been characterized in the Allt a'Mharcaidh catchment in the Scottish Cairngorms in order to: (i) assess the spatial and temporal variation in groundwater chemistry; (ii) identify the hydrogeochemical processes regulating its evolution; and (iii) examine the influence of groundwater on the quality and quantity of stream flow. Shallow groundwater in superficial drift deposits is circumneutral (pH∽7·1) and base cation concentrations are enriched compared with precipitation and drainage water from overlying podzolic soils. Modelling with NETPATH suggests that the dominant geochemical processes that account for this are the dissolution of plagioclase, K-feldspar and biotite. Groundwater emerging as springs from weathered granite underlying high altitude (>900 m) alpine soils shows similar characteristics, though weathering rates are lower, probably as a result of reduced residence times and lower temperatures. Chemical hydrograph separation techniques using acid neutralizing capacity (ANC) and Si as tracers show that groundwater is the dominant source of baseflow in the catchment and also buffers the chemistry of stream water at high flows: groundwater may account for as much as 50–60% of annual runoff in the catchment. Climate and land use in the Cairngorms are vulnerable to future changes, which may have major implications for hydrogeological processes in the area. © 1998 John Wiley & Sons, Ltd.

Hydrochemical study of groundwater in recharge area, Wadi Fatimah basin, Saudi Arabia

The present study deals with the hydrochemical characteristics of groundwater in the upper catchment of Wadi Fatimah basin. The analysis of data indicated strongly that the chloride and sulfate in the groundwater are mainly of marine origin, concentrated by high evaporation processes. Flushing is the most important factor that modifies the ionic concentrations and almost stands for the short-term variation in groundwater chemistry. Weathering reactions of the rock-forming minerals take place under relatively high PCO2 condition in water and soil zone; it can be an important long-term neutralization process. It is accounted as a contributor for Ca and Mg ions in the groundwater. Using the chloride method the recharge rate has been estimated at 72 mm yr−1.

Metal Transport in Groundwater Contaminated by Acid Mine Drainage

Acid mine drainage from a rock dump at an abandoned mine in Dalarna, Sweden, has contaminated the groundwater and created a leachate plume characterized by low pH and high concentrations of Fe, SO4, and heavy metals. Groundwater samples have been collected in order to explain the observed spatial variations in groundwater chemistry. The mineralogy of soil samples collected at the water table has been identified by X-ray diffraction analysis In order to determine which processes may be controlling the evolution of the leachate plume, the geochemical mass-transfer model PHREEQE is used to evaluate a possible reaction pathway controlling the reaction of the leachate plume with uncontaminated groundwater and the till aquifer. The results suggest that silicate weathering and goethite precipitation are the processes primarily controlling observed pH, pe, and Fe concentrations in groundwater direct down-gradient of the rock dump. Increases in K, Na, and Al concentrations along the flow path can be attributed to the feldspar weathering while Al and SO4 concentrations further down-gradient can be controlled by the precipitation of AlOHSO4. The attenuation of heavy metal concentrations may be the result of adsorption and coprecipitation with goethite and other Fe xides.

Geochemical evolution of lacustrine brines from variable-scale groundwater circulation

Evaporative groundwater-fed lakes in the glaciated North American Great Plains vary widely in chemistry. A contributing cause is chemical variability of source groundwater intercepted by specific lakes, caused in part by differing depths of groundwater circulation. Aqueous chemical characteristics of 61 lakes and 160 groundwater samples were compared for an area where such lakes are common in eastern Montana-western North Dakota. Results indicate that groundwater chemistry varies according to depth in a similar fashion within different aquifers. Lake water evaporation from initial groundwater solutions typical of three depths was geochemically modeled using PHRQPITZ, based on a Pitzer treatment of activities and equilibria. Results show that chemistry of most lake waters in the study area may correspond to that predicted from evaporation of shallow- and intermediate-depth groundwater, but not of deep groundwater as postulated in some previous investigations. Lakes in shallow surface depressions receive water primarily from shallow (local) groundwater flow; lakes located in deep or broad topographic depressions may additionally receive groundwater from deeper circulation. In the field area studied, relative dominance of anions (sulfate vs. carbonate) in brines is a signature for inferred depth of source. Also diagnostic is the suite of brine salts formed (NaSO 4Mg salts for shallow flow; these plus NaCO 3 salts for intermediate depth flow). Such source signatures will vary from area to area according to depth variations in groundwater chemistry and in stratigraphy. Chemical evolution of lake water is a two-stage process, with a groundwater path (influenced by residence time, depth of circulation, aquifer mineralogy, and related factors) and a surface path (influenced by evaporation rates, lake-aquifer hydraulics, and lake geochemical reactions). Groundwater flow patterns may affect the former set of factors, thereby indirectly controlling lake water chemistry.

Nitrate Reduction in an Unconfined Sandy Aquifer: Water Chemistry, Reduction Processes, and Geochemical Modeling

Nitrate distribution and reduction processes were investigated in an unconfined sandy aquifer of Quaternary age. Ground water chemistry was studied in a series of eight multilevel samplers along a flow line, deriving water from both arable and forested land. Results show that plumes of nitrate‐contaminated groundwater emanate from the agricultural areas and spread through the aquifer. The aquifer can be subdivided into an upper 10‐ to 15‐m thick oxic zone that contains O2 and NO3−, and a lower anoxic zone characterized by Fe2+‐rich waters. The redox boundary is very sharp, which suggests that reduction processes of O2 and NO3− occur at rates that are fast compared to the rate of downward water transport. Nitrate‐contaminated groundwater contains total contents of dissolved ions that are two to four times higher than in groundwater derived from the forested area. The persistence of the high content of total dissolved ions in the NO3−‐free anoxic zone indicates the downward migration of contaminants and that active nitrate reduction is taking place. Nitrate is apparently reduced to N2 because both nitrite and ammonia are absent or found at very low concentrations. Possible electron donors in the reduced zone of the aquifer are organic matter, present as reworked brown coal fragments from the underlying Miocene, and small amounts of pyrite at an average concentration of 3.6 mmol/kg. Electron balances across the redoxcline, based on concentrations of O2, NO3−, SO42− and total inorganic carbon (TIC), indicate that pyrite is by far the dominant electron donor even though organic matter is much more abundant. Groundwater transport and chemical reactions were modeled using the code PHREEQM, which combines a chemical equilibrium model with a one‐dimensional mixing cell transport model. Only the vertical component of the water transport was modeled since, in contrast to rates along flow lines, the vertical rates are close to constant as required by the one‐dimensional model. Average vertical transport rates of water in the saturated zone were obtained by tritium dating. The modeling process is a two‐step procedure. First the sediment column is initialized with natural water containing only oxygen as electron acceptor, and subsequently agricultural waters containing both oxygen and nitrate are fed into the column. The nitrate concentration of agricultural waters entering the saturated zone varies with time, and an input function was therefore constructed by linear mixing of natural waters and agricultural waters. This input function was fed into the column initialized with natural water, and the model run forward in time to the year 1988 where field data are available. Comparison with field data shows that the variation in groundwater chemistry is well described by the model when reduction of oxygen and reduction of nitrate by pyrite oxidation are the only redox reactions occurring. Finally, predictions are made for the distribution of water chemistry in the year 2003. Downward progression of the redoxcline is accelerated by a factor of five due to nitrate pollution of the aquifer, but absolute rates remain small, of the order of a few centimeters per year. The controlling factor for nitrate migration through the aquifer, once it has reached the anoxic zone, is the concentration and distribution of pyrite in the sediments.

Sawtooth Zoned Calcite Cements in Dinantian Grainstones from South Wales (U.K.)

ABSTRACT Well developed, early meteoric, nonferroan and slightly ferroan calcite cements in the Oolite Group (Dinantian) of South Wales show complex patterns of compositional variation in cathodoluminescence. Crystals contain both sector and multiple concentric zones which preserve a history of continuous morphological change during growth. In addition to relatively rapid transitions between the forms developed at different stages of growth, many of the crystal faces developed an undulose topography, and other crystals appear to contain growth faults. These two features are due to the development of striated faces on which the topographic irregularities parallel possible crystal faces. It is suggested that the spacing of striations should decrease with increasing supersaturation, and thus flat faces (at the optical scale of resolution) will develop both at the lowest and highest degrees of supersaturation. The latter case might often be distinguished from the former by the additional presence of oscillatory zonation. The number and spacing of concentric growth zones suggest that they reflect seasonal fluctuations in porewater chemistry. Locally, where oscillatory zonation is superimposed on broader concentric growth zones, growth appears to have occurred in a series of rapid pulses, also interpreted to reflect seasonal variations in groundwater chemistry.

Lateral and vertical variations in pedogenesis and other early diagenetic phenomena, Middle Jurassic Ravenscar Group, Yorkshire

SUMMARY The effects of early diagenetic processes are widespread in shallow marine and non-marine (deltaic) sediments of the Ravenscar Group. These can be related to both lateral variations in ground water chemistry within the original depositional environment on the lower coastal plain, and to the introduction of sea water into the non-marine sediments and of fresh water into the marine sediments. In the non-marine sediments lateral (catena) variations in soil profiles can be related to floodplain aggradation and abandonment. Near the major channel sands in the Saltwick Formation at Whitby West Cliff, a ganister and seatearth formed in elevated and well drained sediments that were frequently flushed with fresh water. In more distal floodplain areas, ground-waters were influenced by fresh water, but remained generally suboxic, as a result of which siderite cementation occurred. The influence of meteoric water is indicated by the isotopic composition of the cements that formed ( δ 13 C PDB −3.7 to −9.6 ‰, δ 18 O PDB −6.0 to −8.5 ‰). Rhombic siderite cemented the sands, and siderite nodules formed in both sands and shales. Siderite spheruliths formed within more distal soil horizons (such as the Long Nab Member at Crook Ness) where bacterial processes dominated the supply of bicarbonate into the ground water, and in non-marine sediments on abandoned and subsiding delta lobes. Bacterial influence is reflected in the more negative carbon isotopic composition relative to that of the nodules ( δ 13 C PDB −16.2 ‰, δ 18 O PDB −3.5 ‰). Sea water was introduced onto the floodplain locally, providing sulphate ions that were reduced and incorporated into the pyrite that precipitated in the siderite soil horizons. In the marine Millepore Bed sediments at Yons Nab early diagenetic, possibly aragonite, fringes formed on bioclasts and ooliths. These were subsequently deformed by the weight of overburden following renewed progradation. Grain-to-grain contact dissolution and kaolinite precipitation occurred as the fresh groundwater invaded these marine carbonates from the prograding delta above.