Ground waters of the Houston-Galveston Area: Chemical character and industrial utility
No abstract available.
Texas↗
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Stable isotope systematics in mesozoic granites of Central and Northern California and Southwestern Oregon
18O, D, and H2O+ contents were measured for whole-rock specimens of granitoid rocks from 131 localitics in California and southwestern Oregon. With 41 new determinations in the Klamath Mountains and Sierra Nevada, initial strontium isotope ratios are known for 104 of these samples. Large variations in ??18O (5.5 to 12.4), ??D (-130 to -31), water contents (0.14 to 2.23 weight percent) and initial strontium isotope ratios (0.7028 to 0.7095) suggest a variety of source materials and identify rocks modified by secondary processes. Regular patterns of variation in each isotopic ratio exist over large geographical regions, but correlations between the ratios are generally absent except in restricted areas. For example, the regular decrease in ??D values from west to east in the Sierra Nevada batholith is not correlative with a quite complex pattern of ??18O values, implying that different processes were responsible for the isotopic variations in these two elements. In marked contrast to a good correlation between (87Sr/86Sr)o and ??18O observed in the Peninsular Ranges batholith to the south, such correlations are lacking except in a few areas. ??D values, on the other hand, correlate well with rock types, chemistry, and (87Sr/86Sr)o except in the Coast Ranges where few of the isotopic signatures are primary. The uniformly low ??D values of samples from the Mojave Desert indicate that meteoric water contributed much of the hydrogen to the rocks in that area. Even so, the ??18O values and 18O fractionations between quartz and feldspar are normal in these same rocks. This reconnaissance study has identified regularities in geochemical parameters over enormous geographical regions. These patterns are not well understood but merit more detailed examination because they contain information critical to our understanding of the development of granitoid batholiths. ?? 1981 Springer-Verlag.
Contributions to Mineralogy and Petrology↗
Modeling hot spring chemistries with applications to martian silica formation
Many recent studies have implicated hydrothermal systems as the origin of martian minerals across a wide range of martian sites. Particular support for hydrothermal systems include silica (SiO 2 ) deposits, in some cases >90% silica, in the Gusev Crater region, especially in the Columbia Hills and at Home Plate. We have developed a model called CHEMCHAU that can be used up to 100 °C to simulate hot springs associated with hydrothermal systems. The model was partially derived from FREZCHEM, which is a colder temperature model parameterized for broad ranges of temperature (<−70 to 25 °C), pressure (1–1000 bars), and chemical composition. We demonstrate the validity of Pitzer parameters, volumetric parameters, and equilibrium constants in the CHEMCHAU model for the Na–K–Mg–Ca–H–Cl–ClO 4 –SO 4 –OH–HCO 3 –CO 3 –CO 2 –O 2 –CH 4 –Si–H 2 O system up to 100 °C and apply the model to hot springs and silica deposits. A theoretical simulation of silica and calcite equilibrium shows how calcite is least soluble with high pH and high temperatures, while silica behaves oppositely. Such influences imply that differences in temperature and pH on Mars could lead to very distinct mineral assemblages. Using measured solution chemistries of Yellowstone hot springs and Icelandic hot springs, we simulate salts formed during the evaporation of two low pH cases (high and low temperatures) and a high temperature, alkaline (high pH) sodic water. Simulation of an acid-sulfate case leads to precipitation of Fe and Al minerals along with silica. Consistency with martian mineral assemblages suggests that hot, acidic sulfate solutions are plausibility progenitors of minerals in the past on Mars. In the alkaline pH (8.45) simulation, formation of silica at high temperatures (355 K) led to precipitation of anhydrous minerals (CaSO 4 , Na 2 SO 4 ) that was also the case for the high temperature (353 K) low pH case where anhydrous minerals (NaCl, CaSO 4 ) also precipitated. Thus we predict that secondary minerals associated with massive silica deposits are plausible indicators on Mars of precipitation environments and aqueous chemistry. Theoretical model calculations are in reasonable agreement with independent experimental silica concentrations, which strengthens the validity of the new CHEMCHAU model.
Icarus↗
Accumulation of pesticides in pacific chorus frogs (Pseudacris regilla) from California's Sierra Nevada Mountains, USA
Pesticides are receiving increasing attention as potential causes of amphibian declines, acting singly or in combination with other stressors, but limited information is available on the accumulation of current-use pesticides in tissue. The authors examined potential exposure and accumulation of currently used pesticides in pond-breeding frogs (Pseudacris regilla) collected from 7 high elevations sites in northern California. All sites sampled are located downwind of California's highly agricultural Central Valley and receive inputs of pesticides through precipitation and/or dry deposition. Whole frog tissue, water, and sediment were analyzed for more than 90 current-use pesticides and pesticide degradates using gas chromatography–mass spectrometry. Two fungicides, pyraclostrobin and tebuconazole, and one herbicide, simazine, were the most frequently detected pesticides in tissue samples. Median pesticide concentration ranged from 13 µg/kg to 235 µg/kg wet weight. Tebuconazole and pyraclostrobin were the only 2 compounds observed frequently in frog tissue and sediment. Significant spatial differences in tissue concentration were observed, which corresponded to pesticide use in the upwind counties. Data generated indicated that amphibians residing in remote locations are exposed to and capable of accumulating current-use pesticides. A comparison of P. regilla tissue concentrations with water and sediment data indicated that the frogs are accumulating pesticides and are potentially a more reliable indicator of exposure to this group of pesticides than either water or sediment.
California↗
Oxygen isotopes in nitrite: Analysis, calibration, and equilibration
Nitrite is a central intermediate in the nitrogen cycle and can persist in significant concentrations in ocean waters, sediment pore waters, and terrestrial groundwaters. To fully interpret the effect of microbial processes on nitrate (NO 3 - ), nitrite (NO 2 - ), and nitrous oxide (N 2 O) cycling in these systems, the nitrite pool must be accessible to isotopic analysis. Furthermore, because nitrite interferes with most methods of nitrate isotopic analysis, accurate isotopic analysis of nitrite is essential for correct measurement of nitrate isotopes in a sample that contains nitrite. In this study, nitrite salts with varying oxygen isotopic compositions were prepared and calibrated and then used to test the denitrifier method for nitrite oxygen isotopic analysis. The oxygen isotopic fractionation during nitrite reduction to N 2 O by Pseudomonas aureofaciens was lower than for nitrate conversion to N 2 O, while oxygen isotopic exchange between nitrite and water during the reaction was similar. These results enable the extension of the denitrifier method to oxygen isotopic analysis of nitrite (in the absence of nitrate) and correction of nitrate isotopes for the presence of nitrite in “mixed” samples. We tested storage conditions for seawater and freshwater samples that contain nitrite and provide recommendations for accurate oxygen isotopic analysis of nitrite by any method. Finally, we report preliminary results on the equilibrium isotope effect between nitrite and water, which can play an important role in determining the oxygen isotopic value of nitrite where equilibration with water is significant.
Analytical Chemistry↗
Transport of chromium and selenium in a pristine sand and gravel aquifer: Role of adsorption processes
Field transport experiments were conducted in an oxic sand and gravel aquifer using Br (bromide ion), Cr (chromium, injected as Cr(VI)), Se (selenium, injected as Se(VI)), and other tracers. The aquifer has mildly acidic p H values and low concentrations of dissolved salts. Within analytical errors, all mobile Cr was present as Cr(VI). All mobile Se was probably present as Se(VI). Adsorption of Cr and Se onto aquifer sediments caused retardation of both tracers. Breakthrough curves for Cr and Se had extensive tails, which caused large decreases in their maximum concentrations relative to the nonreactive Br tracer after only 2.0 m of transport. A surface complexation model was applied to the results of laboratory studies of Cr(VI) adsorption on aquifer solids from the site based on adsorption onto hydrous ferric oxide. The modeling results suggested that the dominant adsorbents in the aquifer solids have lower affinities for anion adsorption than pure hydrous ferric oxide. The steep rising limbs and extensive tails observed in most of the breakthrough curves are qualitatively consistent with the equilibrium surface complexation model; however, slow rates of adsorption and desorption may have contributed to these features. Variations during transport in the concentrations of Cr, Se, and other anions competing for adsorption sites likely gave rise to variations in the extent of adsorption. Adequate description of the observed retardation of Cr and Se would require a coupled transport-adsorption model that can account for these effects. Companion experiments in the mildly reducing zone of the aquifer (Kent et al., 1994) showed a loss of Cr mass, probably resulting from reduction to Cr(III), and little retardation of mobile Cr and Se during transport; this contrast illustrates the influence of aquifer chemistry on the transport of redox-sensitive solutes.
Water Resources Research↗
Chinese tallow: Invading the southeastern Coastal Plain
Chinese tallow is an ornamental tree with colorful autumn foliage that can survive full sunlight and shade, flooding, drought, and in some cases fire. To horticulturists this kind of tree sounds like a dream, but to ecologists, land managers, and land owners this kind of tree can be a nightmare, especially when it invades an area and takes over native vegetation. Chinese tallow ( Triadica sebifera ), a nonnative tree from China, is currently transforming the southeastern Coastal Plain. Over the last 30 years, Chinese tallow has become a common tree in old fields and bottomland swamps of coastal Louisiana. Several studies at the U.S. Geological Survey's National Wetlands Research Center (NWRC), Lafayette, Louisiana, are aimed at understanding the factors that contribute to Chinese tallow growth, spread, and management. When tallow invades, it eventually monopolizes an area, creating a forest without native animal or plant species. This tree exhibits classic traits of most nonnative invaders: it is attractive so people want to distribute it, it has incredible resiliency, it grows quickly and in a variety of soils, and it is resistant to pests. In the coastal prairie of Louisiana and Texas, Chinese tallow can grow up to 30 feet and shade out native sun-loving prairie species. The disappearing of prairie species is troublesome because less than 1% of original coastal prairie remains, and in Louisiana, less than 500 of the original 2.2 million acres still exist. Tallow reproduces and grows quickly and can cause large-scale ecosystem modification (fig. 1). For example, when it completely replaces native vegetation, it has a negative effect on birds by degrading the habitat. Besides shading out grasses that cattle like to eat, it can also be potentially harmful to humans and animals because of its berries (fig. 2) and plant sap that contain toxins. There is some concern its leaves may shed toxins that change the soil chemistry and make it difficult for other plants to grow.
Fact Sheet↗
Cadmium, copper, lead, and zinc determination in precipitation: A comparison of inductively coupled plasma atomic emission spectrometry and graphite furnace atomization atomic absorption spectrometry
Selected trace element analysis for cadmium, copper, lead, and zinc in precipitation samples by inductively coupled plasma atomic emission Spectrometry (ICP) and by atomic absorption spectrometry with graphite furnace atomization (AAGF) have been evaluated. This task was conducted in conjunction with a longterm study of precipitation chemistry at high altitude sites located in remote areas of the southwestern United States. Coefficients of variation and recovery values were determined for a standard reference water sample for all metals examined for both techniques. At concentration levels less than 10 micrograms per liter AAGF analyses exhibited better precision and accuracy than ICP. Both methods appear to offer the potential for cost-effective analysis of trace metal ions in precipitation. ?? 1987 Springer-Verlag.
Mikrochimica Acta↗
Weathering of sulfidic shale and copper mine waste: Secondary minerals and metal cycling in Great Smoky Mountains National Park, Tennessee, and North Carolina, USA
Metal cycling via physical and chemical weathering of discrete sources (copper mines) and regional (non-point) sources (sulfide-rich shale) is evaluated by examining the mineralogy and chemistry of weathering products in Great Smoky Mountains National Park, Tennessee, and North Carolina, USA. The elements in copper mine waste, secondary minerals, stream sediments, and waters that are most likely to have negative impacts on aquatic ecosystems are aluminum, copper, zinc, and arsenic because these elements locally exceed toxicity guidelines for surface waters or for stream sediments. Acid-mine drainage has not developed in streams draining inactive copper mines. Acid-rock drainage and chemical weathering processes that accompany debris flows or human disturbances of sulfidic rocks are comparable to processes that develop acid-mine drainage elsewhere. Despite the high rainfall in the mountain range, sheltered areas and intermittent dry spells provide local venues for development of secondary weathering products that can impact aquatic ecosystems.
Environmental Geology↗
Environmental geochemistry of a Kuroko-type massive sulfide deposit at the abandoned Valzinco mine, Virginia, USA
The abandoned Valzinco mine, which worked a steeply dipping Kuroko-type massive sulfide deposit in the Virginia Au-pyrite belt, contributed significant metal-laden acid-mine drainage to the Knight's Branch watershed. The host rocks were dominated by metamorphosed felsic volcanic rocks, which offered limited acid-neutralizing potential. The ores were dominated by pyrite, sphalerite, galena, and chalcopyrite, which represented significant acid-generating potential. Acid-base accounting and leaching studies of flotation tailings - the dominant mine waste at the site - indicated that they were acid generating and therefore, should have liberated significant quantities of metals to solution. Field studies of mine drainage from the site confirmed that mine drainage and the impacted stream waters had pH values from 1.1 to 6.4 and exceeded aquatic ecosystem toxicity limits for Fe, Al, Cd, Cu, Pb and Zn. Stable isotope studies of water, dissolved SO42 -, and primary and secondary sulfate and sulfide minerals indicated that two distinct sulfide oxidation pathways were operative at the site: one dominated by Fe(III) as the oxidant, and another by molecular O2 as the oxidant. Reaction-path modeling suggested that geochemical interactions between tailings and waters approached a steady state within about a year. Both leaching studies and geochemical reaction-path modeling provided reasonable predictions of the mine-drainage chemistry.
Applied Geochemistry↗
Mapping playa evaporite minerals and associated sediments in Death Valley, California, with multispectral thermal infrared images
Efflorescent salt crusts and associated sediments in Death Valley, California, were studied with remote-sensing data acquired by the NASA thermal infrared multispectral scanner (TIMS). Nine spectral classes that represent a variety of surface materials were distinguished, including several classes that reflect important aspects of the playa groundwater chemistry and hydrology. Evaporite crusts containing abundant thenardite (sodium sulfate) were mapped along the northern and eastern margins of the Cottonball Basin, areas where the inflow waters are rich in sodium. Gypsum (calcium sulfate) crusts were more common in the Badwater Basin, particularly near springs associated with calcic groundwaters along the western basin margin. Evaporite-rich crusts generally marked areas where groundwater is periodically near the surface and thus able to replenish the crusts though capillary evaporation. Detrital silicate minerals were prevalent in other parts of the salt pan where shallow groundwater does not affect the surface composition. The surface features in Death Valley change in response to climatic variations on several different timescales. For example, salt crusts on low-lying mudflats form and redissolve during seasonal-to-interannual cycles of wetting and desiccation. In contrast, recent flooding and erosion of rough-salt surfaces in Death Valley probably reflect increased regional precipitation spanning several decades. Remote-sensing observations of playas can provide a means for monitoring changes in evaporite facies and for better understanding the associated climatic processes. At present, such studies are limited by the availability of suitable airborne scanner data. However, with the launch of the Earth Observing System (EOS) AM-1 Platform in 1998, multispectral visible/near-infrared and thermal infrared remote-sensing data will become globally available.
Journal of Geophysical Research B: Solid Earth↗
Spectroscopy from Space
This chapter reviews detection of materials on solid and liquid (lakes and ocean) surfaces in the solar system using ultraviolet to infrared spectroscopy from space, or near space (high altitude aircraft on the Earth), or in the case of remote objects, earth-based and earth-orbiting telescopes. Point spectrometers and imaging spectrometers have been probing the surfaces of our solar system for decades. Spacecraft carrying imaging spectrometers are currently in orbit around Mercury, Venus, Earth, Mars, and Saturn, and systems have recently visited Jupiter, comets, asteroids, and one spectrometer-carrying spacecraft is on its way to Pluto. Together these systems are providing a wealth of data that will enable a better understanding of the composition of condensed matter bodies in the solar system. Minerals, ices, liquids, and other materials have been detected and mapped on the Earth and all planets and/or their satellites where the surface can be observed from space, with the exception of Venus whose thick atmosphere limits surface observation. Basaltic minerals (e.g., pyroxene and olivine) have been detected with spectroscopy on the Earth, Moon, Mars and some asteroids. The greatest mineralogic diversity seen from space is observed on the Earth and Mars. The Earth, with oceans, active tectonic and hydrologic cycles, and biological processes, displays the greatest material diversity including the detection of amorphous and crystalline inorganic materials, organic compounds, water and water ice. Water ice is a very common mineral throughout the Solar System and has been unambiguously detected or inferred in every planet and/or their moon(s) where good spectroscopic data has been obtained. In addition to water ice, other molecular solids have been observed in the solar system using spectroscopic methods. Solid carbon dioxide is found on all systems beyond the Earth except Pluto, although CO 2 sometimes appears to be trapped in other solids rather than as an ice on some objects. The largest deposits of carbon dioxide ice are found on Mars. Sulfur dioxide ice is found in the Jupiter system. Nitrogen and methane ices are common beyond the Uranian system. Saturn’s moon Titan probably has the most complex active extra-terrestrial surface chemistry involving organic compounds. Some of the observed or inferred compounds include ices of benzene (C 6 H 6 ), cyanoacetylene (HC 3 N), toluene (C 7 H 8 ), cyanogen (C 2 N 2 ), acetonitrile (CH 3 CN), water (H 2 O), carbon dioxide (CO 2 ), and ammonia (NH 3 ). Confirming compounds on Titan is hampered by its thick smoggy atmosphere, where in relative terms the atmospheric interferences that hamper surface characterization lie between that of Venus and Earth. In this chapter we exclude discussion of the planets Jupiter, Saturn, Uranus, and Neptune because their thick atmospheres preclude observing the surface, even if surfaces exist. However, we do discuss spectroscopic observations on a number of the extra-terrestrial satellite bodies. Ammonia was predicted on many icy moons but is notably absent among the definitively detected ices with possible exceptions on Charon and possible trace amounts on some of the Saturnian satellites. Comets, storehouses of many compounds that could exist as ices in their nuclei, have only had small amounts of water ice definitively detected on their surfaces from spectroscopy. Only two asteroids have had a direct detection of surface water ice, although its presence can be inferred in others.
Reviews in Mineralogy and Geochemistry↗
Mass transfer and carbon isotope evolution in natural water systems
This paper presents a theoretical treatment of the evolution of the carbon isotopes C13 and C14 in natural waters and in precipitates which derive from such waters. The effects of an arbitrary number of sources (such as dissolution of carbonate minerals and oxidation of organic material) and sinks (such as mineral precipitation, CO2 degassing and production of methane), and of equilibrium fractionation between solid, gas and aqueous phases are considered. The results are expressed as equations relating changes in isotopic composition to changes in conventional carbonate chemistry. One implication of the equations is that the isotopic composition of an aqueous phase may approach a limiting value whenever there are simultaneous inputs and outputs of carbonate. In order to unambiguously interpret isotopic data from carbonate precipitates and identify reactants and products in reacting natural waters, it is essential that isotopic changes are determined chiefly by reactant and product stoichiometry, independent of reaction path. We demonstrate that this is so by means of quantitative examples. The evolution equations are applied to: 1. (1) carbon-14 dating of groundwaters; 2. (2) interpretation of the isotopic composition of carbonate precipitates, carbonate cements and diagenetically altered carbonates; and 3. (3) the identification of chemical reaction stoichiometry. These applications are illustrated by examples which show the variation of ??C13 in solutions and in precipitates formed under a variety of conditions involving incongruent dissolution, CO2 degassing, methane production and mineral precipitation. ?? 1978.
Geochimica et Cosmochimica Acta↗
Determination of alachlor and its sulfonic acid metabolite in water by solid-phase extraction and enzyme-linked immunosorbent assay
No abstract available.
Analytical Chemistry↗
Sources of error in the use in water analysis of Fairchild's methode for determination of fluoride in phosphate rock
No abstract available.
Industrial And Engineering Chemistry Analytical Ed↗
Chemistry, mineralogy and origin of the clay-hill nitrate deposits, Amargosa River valley, Death Valley region, California, U.S.A.
The clay-hill nitrate deposits of the Amargosa River valley, California, are caliche-type accumulations of water-soluble saline minerals in clay-rich soils on saline lake beds of Miocene, Pliocene(?) and Pleistocene age. The soils have a maximum thickness of ??? 50 cm, and commonly consist of three layers: (1) an upper 5-10 cm of saline-free soil; (2) an underlying 15-20 cm of rubbly saline soil; and (3) a hard nitrate-rich caliche, 10-20 cm thick, at the bottom of the soil profile. The saline constituents, which make up as much as 50% of the caliche, are chiefly Cl-, NO-3, SO2-4 and Na+. In addition are minor amounts of K+, Mg2+ and Ca2+, varying, though generally minor, amounts of B2O3 and CO2-3, and trace amounts of I (probably as IO-3), NO-2, CrO2-4 and Mo (probably as MoO2-4). The water-soluble saline materials have an I/Br ratio of ??? 1, which is much higher than nearly all other saline depostis. The principal saline minerals of the caliche are halite (NaCl), nitratite (NaNO3), darapskite (Na3(SO4)(NO3)??H2O), glauberite (Na2Ca(SO4)2), gypsum (CaSO4??2H2O) and anhydrite (CaSO4). Borax (Na2B4O5(OH)4??8H2O), tincalconite (Na2B4O5(OH)4??3H2O) and trona (Na3(CO3)(HCO3)??2H2O) are abundant locally. The clay-hill nitrate deposits are analogous to the well-known Chilean nitrate deposits, and probably are of similar origin. Whereas the Chilean deposits are in permeable soils of the nearly rainless Atacama Desert, the clay-hill deposits are in relatively impervious clay-rich soils that inhibited leaching by rain water. The annual rainfall in the Death Valley region of ??? 5 cm is sufficient to leach water-soluble minerals from the more permeable soils. The clay-hill deposits contain saline materials from the lake beds beneath the nitrate deposits are well as wind-transported materials from nearby clay-hill soils, playas and salt marshes. The nitrate is probably of organic origin, consisting of atmospheric nitrogen fixed as protein by photoautotrophic blue-green algae, which are thought to form crusts on soils at the sites of the deposits when moistened by rainfall. The protein is subsequently transformed to nitrate by autotophic bacteria. ?? 1988.
Chemical Geology↗
Land development and road salt usage drive long-term changes in major-ion chemistry of streamwater in six exurban and suburban watersheds, southeastern Pennsylvania, 1999-2019
In urbanized areas, the “freshwater salinization syndrome” (FSS), which pertains to long-term increases in concentrations of major ions and metals in fresh surface waters, has been attributed to road salt application. In addition to FSS, the water composition changes as an influx of sodium (Na + ) in recharge may displace calcium (Ca 2+ ), magnesium (Mg 2+ ), potassium (K + ), and trace metals by reverse cation exchange. These changing ion fluxes can result in adverse impacts on groundwater and surface waters used for municipal supplies. Few datasets exist to quantify the FSS on a watershed scale or link its manifestation to potential controlling factors such as changes in urban development, land use/land cover (LULC), or wastewater treatment plant (WWTP) discharges in upstream areas. Here, we use two decades (1999–2019) of monthly streamwater quality data combined with daily streamflow for six exurban and suburban watersheds in southeastern Pennsylvania to examine the relations among Ca 2+ , Mg 2+ , K + , Na + , chloride (Cl − ), sulfate (SO 4 2- ), and alkalinity (HCO 3 − ) concentrations and upstream controlling factors. Flow-normalized annual and baseflow (August ̶ November) concentrations for Ca 2+ , Mg 2+ , Na + , and Cl − increased in all six watersheds over the 20-year study, providing evidence of FSS’s impacts on groundwater that sustains streamflow. Additionally, a redundancy analysis using 2019 flow-normalized values identified the following positive associations between solute concentrations and controlling variables: 1) Cl − , Mg 2+ , and Ca 2+ with impervious surface cover (ISC), 2) Na + and SO 4 2- with ISC and total WWTP discharge volume, and 3) HCO 3 − with agriculture and total WWTP discharge volume. From a human health perspective, 2019 flow-normalized Na + concentrations exceeded the U.S. Environmental Protection Agency’s 20 mg L -1 threshold for individuals restricted to a low sodium diet. Furthermore, indices used to evaluate the corrosivity of source waters to drinking water infrastructure and inform municipal water treatment practices, such as the Chloride to Sulfate Mass Ratio and Larson Ratio, increased between two- and seven-fold over the 20-year time. Collectively, the results elucidate the causal factors of the FSS in suburban and exurban watersheds and its potential impacts on human health and drinking water infrastructure.
Pennsylvania↗
Depth-dependent sampling to identify short-circuit pathways to public-supply wells in multiple aquifer settings in the United States
Depth-dependent water-quality and borehole flow data were used to determine where and how contamination enters public-supply wells (PSWs) at study sites in different principal aquifers of the United States. At each of three study sites, depth-dependent samples and wellbore flow data were collected from multiple depths in selected PSWs under pumping conditions. The chemistry of these depth-dependent samples, along with samples of the surface discharge from the PSWs, was compared to that of adjacent nested monitoring wells. The results of depth-dependent analyses from sites in Modesto (California), York (Nebraska), and Tampa (Florida) are summarized and compared. Although the exact mechanisms for transport of contaminants to the PSWs varied among these hydrogeologic settings, in all three settings the presence of wells or boreholes or natural preferential flow paths allowed water and contaminants to bypass substantial portions of the aquifer and to reach PSWs or depths in the aquifer more quickly than would have occurred in the absence of these short-circuiting flow paths. The chemistry and flow data from multiple depths was essential to developing an understanding of the dominant flow paths of contaminants to PSW in all three settings. This knowledge contributes to developing effective strategies for monitoring and protection.
Hydrogeology Journal↗