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Search indexed USGS publications on groundwater, aquifers, geologic maps, mineral resources and earthquakes. Explore source records by subject and place.

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At least 1,675 records · Page 93Linked to original sources

USGS1987

Monodisperse ferrous phosphate colloids in an anoxic groundwater plume

Groundwater samples collected near a secondary-sewage infiltration site on Cape Cod, Massachusetts were examined for colloidal materials (10–1000 nm). In two wells the water contained a population of monodisperse 100-nm particles, detected using laser-light scattering and autocorrelation data processing. SEM and SEM-EDAX analysis of these colloidal materials collected on ultrafilters confirmed the laser light scattering result and revealed that these microparticles consisyed of primarily iron and phosphorus in a 1.86 Fe to 1.0 P stoichiometric ratio. Chemical analyses of the water samples, together with equilibrium solubility calculations, strongly suggest that the ion-activity product should exceed the solubility product of a 100-nm diameter predominantly vivianite-type (Fe 3 (PO 4 ) 2 · 8H 2 O) colloidal phase. In light of our results, we conclude that these microparticles were formed by sewage-derived phosphate combining with ferrous iron released from the aquifer solids, and that these colloids may be moving in the groundwater flow. Such a subsurface transport process could have major implications regarding the movement of particle-reactive pollutants traditionally viewed as non-mobile in groundwater.

Massachusetts↗

USGS1996

Simulation of phosphate transport in sewage-contaminated groundwater, Cape Cod, Massachusetts

Sewage-contaminated groundwater currently discharges to Ashumet Pond, located on Cape Cod, Massachusetts. Phosphate concentrations as high as 60 μmol l −1 have been measured in groundwater entering Ashumet Pond, and there is concern that the rate of eutrophication could increase. Phosphate in the sewage plume is sorbed by aquifer sediment; the amount is a function of phosphate concentration and pH. A nonelectrostatic surface-complexation model coupled with a one-dimensional solute-transport code was used to simulate sorption and desorption of phosphate in laboratory column experiments. The model simulated sorption of phosphate reasonably well, although the slow rate of approach to complete breakthrough indicated a nonequilibrium process that was not accounted for in the solute-transport model. The rate of phosphate desorption in the column experiments was relatively slow. Phosphate could still be measured in effluent after 160 pore volumes of uncontaminated groundwater had been flushed through the columns. Desorption was partly a function of the slowly decreasing pH in the columns and could be modeled quantitatively. Disposal of sewage at this site is scheduled to stop in 1995; however, a large reservoir of sorbed phosphate exists on aquifer sediment upgradient from Ashumet Pond. Computer simulations predict that desorption of phosphate could result in contamination of Ashumet Pond for decades.

Applied Geochemistry↗

USGS2010

Groundwater nutrient concentrations during prairie reconstruction on an Iowa landscape

One anticipated benefit of ecosystem restoration is water quality improvement. This study evaluated NO3-N and phosphorus in subsurface waters during prairie establishment following decades of row-crop agriculture. A prairie seeding in late 2003 became established in 2006. Wells and suction cup samplers were monitored for NO3-N and phosphorus. Nitrate-N varied with time and landscape position. Non-detectable NO3-N concentrations became modal along ephemeral drainageways in 2006, when average concentrations in uplands first became <10mg NO3-NL-1. This decline continued and upland groundwater averaged near 2mg NO3-NL-1 after 2007. The longer time lag in NO3-N response in uplands was attributed to greater quantities of leachable N in upland subsoils. Spatial differences in vadose-zone travel times were less important, considering water table dynamics. Phosphorus showed a contrasting landscape pattern, without any obvious temporal trend. Phosphorus was greatest along and near ephemeral drainageways. Sediment accumulation from upland agricultural erosion provided a source of P along drainageways, where shallow, reductive groundwater increased P solubility. Phosphorus exceeded eutrophication risk thresholds in these lower areas, where saturation-excess runoff could readily transport P to surface waters. Legacy impacts of past agricultural erosion and sedimentation may include soluble phosphorus in shallow groundwater, at sites prone to saturation-excess runoff. ?? 2010.

Agriculture, Ecosystems and Environment↗

USGS2015

Insights into controls on hexavalent chromium in groundwater provided by environmental tracers, Sacramento Valley, California, USA

Environmental tracers are useful for determining groundwater age and recharge source, yet their application in studies of geogenic Cr(VI) in groundwater has been limited. Environmental tracer data from 166 wells located in the Sacramento Valley, northern California, were interpreted and compared to Cr concentrations to determine the origin and age of groundwater with elevated Cr(VI), and better understand where Cr(VI) becomes mobilized and how it evolves along flowpaths. In addition to major ion and trace element concentrations, the dataset includes δ 18 O, δ 2 H, 3 H concentration, 14 C activity (of dissolved inorganic C), δ 13 C, 3 He/ 4 He ratio, and noble gas concentrations (He, Ne, Ar, Kr, Xe). Noble gas recharge temperatures (NGTs) were computed, and age-related tracers were interpreted in combination to constrain the age distribution in samples and sort them into six different age categories spanning from <60 yr old to >10,000 yr old. Nearly all measured Cr is in the form of Cr(IV). Concentrations range from <1 to 46 μg L −1 , with 10% exceeding the state of California’s Cr(VI) maximum contaminant level of 10 μg L −1 . Two groups with elevated Cr(VI) (⩾5 μg L −1 ) were identified. Group 1 samples are from the southern part of the valley and contain modern (<60 yr old) water, have elevated NO 3 − concentrations (>3 mg L −1 ), and commonly have δ 18 O values enriched relative to local precipitation. These samples likely contain irrigation water and are elevated due to accelerated mobilization of Cr(VI) in the unsaturated zone (UZ) in irrigated areas. Group 2 samples are from throughout the valley and typically contain water 1000–10,000 yr old, have δ 18 O values consistent with local precipitation, and have unexpectedly warm NGTs. Chromium(VI) concentrations in Group 2 samples may be elevated for multiple reasons, but the hypothesis most consistent with all available data (notably, the warm NGTs) is a relatively long UZ residence time due to recharge through a deep UZ near the margin of the basin. A possible explanation for why Cr(VI) may be primarily mobilized in the UZ rather than farther along flowpaths in the oxic portion of the saturated zone is more dynamic cycling of Mn in the UZ due to transient moisture and redox conditions.

California↗

USGS2016

Use of multiple age tracers to estimate groundwater residence times and long-term recharge rates in arid southern Oman

Multiple age tracers were measured to estimate groundwater residence times in the regional aquifer system underlying southwestern Oman. This area, known as the Najd, is one of the most arid areas in the world and is planned to be the main agricultural center of the Sultanate of Oman in the near future. The three isotopic age tracers 4 He, 14 C and 36 Cl were measured in waters collected from wells along a line that extended roughly from the Dhofar Mountains near the Arabian Sea northward 400 km into the Empty Quarter of the Arabian Peninsula. The wells sampled were mostly open to the Umm Er Radhuma confined aquifer, although, some were completed in the mostly unconfined Rus aquifer. The combined results from the three tracers indicate the age of the confined groundwater is < 40 ka in the recharge area in the Dhofar Mountains, > 100 ka in the central section north of the mountains, and up to and > one Ma in the Empty Quarter. The 14 C data were used to help calibrate the 4 He and 36 Cl data. Mixing models suggest that long open boreholes north of the mountains compromise 14 C-only interpretations there, in contrast to 4 He and 36 Cl calculations that are less sensitive to borehole mixing. Thus, only the latter two tracers from these more distant wells were considered reliable. In addition to the age tracers, δ 2 H and δ 18 O data suggest that seasonal monsoon and infrequent tropical cyclones are both substantial contributors to the recharge. The study highlights the advantages of using multiple chemical and isotopic data when estimating groundwater travel times and recharge rates, and differentiating recharge mechanisms.

Dhofar Mountains↗

USGS2013

Revision of Fontes & Garnier's model for the initial 14 C content of dissolved inorganic carbon used in groundwater dating

The widely applied model for groundwater dating using 14 C proposed by Fontes and Garnier (F&G) (Fontes and Garnier, 1979) estimates the initial 14 C content in waters from carbonate-rock aquifers affected by isotopic exchange. Usually, the model of F&G is applied in one of two ways: (1) using a single 13 C fractionation factor of gaseous CO 2 with respect to a solid carbonate mineral, εg/s, regardless of whether the carbon isotopic exchange is controlled by soil CO 2 in the unsaturated zone, or by solid carbonate mineral in the saturated zone; or (2) using different fractionation factors if the exchange process is dominated by soil CO 2 gas as opposed to solid carbonate mineral (typically calcite). An analysis of the F&G model shows an inadequate conceptualization, resulting in underestimation of the initial 14 C values ( 14 C 0 ) for groundwater systems that have undergone isotopic exchange. The degree to which the 14 C 0 is underestimated increases with the extent of isotopic exchange. Examples show that in extreme cases, the error in calculated adjusted initial 14 C values can be more than 20% modern carbon (pmc). A model is derived that revises the mass balance method of F&G by using a modified model conceptualization. The derivation yields a “global” model both for carbon isotopic exchange dominated by gaseous CO 2 in the unsaturated zone, and for carbon isotopic exchange dominated by solid carbonate mineral in the saturated zone. However, the revised model requires different parameters for exchange dominated by gaseous CO 2 as opposed to exchange dominated by solid carbonate minerals. The revised model for exchange dominated by gaseous CO 2 is shown to be identical to the model of Mook (Mook, 1976). For groundwater systems where exchange occurs both in the unsaturated zone and saturated zone, the revised model can still be used; however, 14 C 0 will be slightly underestimated. Finally, in carbonate systems undergoing complex geochemical reactions, such as oxidation of organic carbon, radiocarbon ages are best estimated by inverse geochemical modeling techniques.

Chemical Geology↗

USGS2015

Substantial nitrous oxide emissions from intertidal sediments and groundwater in anthropogenically-impacted West Falmouth Harbor, Massachusetts

Large N2O emissions were observed from intertidal sediments in a coastal estuary, West Falmouth Harbor, MA, USA. Average N2O emission rates from 41 chambers during summer 2008 were 10.7 mol N2O m(-2) h(-1)±4.43 μmol N2O m(-2) h(-1) (standard error). Emissions were highest from sediments within a known wastewater plume, where a maximum N2O emission rate was 155 μmol N2O m(-2) h(-1). Intertidal N2O fluxes were positively related to porewater ammonium concentrations at 10 and 25 cm depths. In groundwater from 7 shoreline wells, dissolved N2O ranged from 488% of saturation (56 nM N2O) to more than 13000% of saturation (1529 nM N2O) and was positively related to nitrate concentrations. Fresh and brackish porewater underlying 14 chambers was also supersaturated in N2O, ranging from 2980% to 13175% of saturation. These observations support a relationship between anthropogenic nutrient loading and N2O emissions in West Falmouth Harbor, with both groundwater sources and also local N2O production within nutrient-rich, intertidal sediments in the groundwater seepage face. N2O emissions from intertidal "hotspot" in this harbor, together with estimated surface water emissions, constituted 2.4% of the average overall rate of nitrogen export from the watershed to the estuary. This suggests that N2O emissions factors from coastal ecosystems may be underestimated. Since anthropogenic nutrient loading affects estuaries worldwide, quantification of N2O dynamics is warranted in other anthropogenically-impacted coastal ecosystems.

Massachusetts↗

USGS2011

Transient changes in shallow groundwater chemistry during the MSU ZERT CO2 injection experiment

Food-grade CO 2 was injected into a shallow aquifer through a perforated pipe placed horizontally 1–2 m below the water table at the Montana State University Zero Emission Research and Technology (MSU-ZERT) field site at Bozeman, Montana. The possible impact of elevated CO 2 levels on groundwater quality was investigated by analyzing 80 water samples taken before, during, and following CO 2 injection. Field determinations and laboratory analyses showed rapid and systematic changes in pH, alkalinity, and conductance, as well as increases in the aqueous concentrations of trace element species. The geochemical data were first evaluated using principal component analysis (PCA) in order to identify correlations between aqueous species. The PCA findings were then used in formulating a geochemical model to simulate the processes likely to be responsible for the observed increases in the concentrations of dissolved constituents. Modeling was conducted taking into account aqueous and surface complexation, cation exchange, and mineral precipitation and dissolution. Reasonable matches between measured data and model results suggest that: (1) CO 2 dissolution in the groundwater causes calcite to dissolve. (2) Observed increases in the concentration of dissolved trace metals result likely from Ca +2 -driven ion exchange with clays (smectites) and sorption/desorption reactions likely involving Fe (hydr)oxides. (3) Bicarbonate from CO 2 dissolution appears to compete for sorption with anionic species such as HAsO 4 −2 , potentially increasing dissolved As levels in groundwater.

Conference Paper↗

USGS2017

Groundwater-derived nutrient and trace element transport to a nearshore Kona coral ecosystem: Experimental mixing model results

Study region The groundwater influenced coastal waters along the arid Kona coast of the Big Island, Hawai’i. Study focus A salinity-and phase partitioning-based mixing experiment was constructed using contrasting groundwater endmembers along the arid Konacoast of the Big Island, Hawai’i and local open seawater to better understand biogeochemical and physicochemical processes that influence the fate of submarine groundwater discharge (SGD)-derived nutrients and trace elements. New Hydrological Insights for the Region Treated wastewater effluent was the main source for nutrient enrichment downstream at the Honokōhau Harbor site. Conservative mixing for some constituents, such as nitrate + nitrite, illustrate the effectiveness of physical mixing to maintain oceanic concentrations in the colloid (0.02–0.45 μm) and truly dissolved (

Hawaii↗

USGS2025

Simulation of the impacts of projected climate change on groundwater resources in the urban, semiarid Yucaipa Valley watershed, southern California using an integrated hydrologic model

Managing water resources in semiarid watersheds is challenging due to limited supply and uncertain future climate conditions. This paper examines the impact of future climate changes on an urban watershed in southern California using an integrated hydrologic model. GSFLOW modeling software is used to simulate the nonlinear relationships between climate trends and precipitation partitioning into ET, runoff, and subsurface storage. Four global circulation models (GCMs), each with two greenhouse-gas scenarios, RCP45 and RCP85 are used to project future climate conditions. GCMs include the CanESM2, CNRM-CM5, HadGEM2-ES, and MIROC5 models. The model's simulated hydrologic conditions are compared with historical data to assess changes in water budgets and groundwater supply. Results indicate decreased groundwater storage in most scenarios due to increased natural evapotranspiration, vegetation consumptive use, and streamflow out of the watershed. Only scenarios with substantially increased future precipitation show increased groundwater storage. The study also highlights increased future aridity despite the rise in precipitation and large precipitation events forecast by GCMs, which increase the risk of urban floods and decrease stream leakage and water available to vegetation.

California↗

USGS2025

Systematic approach to prioritize wells for effective groundwater monitoring and management in the Arkansas Headwaters Basin, Colorado, USA

Study region The Arkansas Headwaters Basin, an intermountain basin in the Southern Rocky Mountains of North America. Study focus Our specific focus is choosing a set of wells to support a possible future regional groundwater-surface water model that would support water management. We present a three-step process using multiple criteria to score, predict, and choose prioritized wells that capture the full distribution of data including extremes. The three-step process provides accessible visualizations, fiscally efficient well prioritization, and screening useful for subsequent groundwater modeling. The novelty of the proposed methodology is the systematic approach integrating a scoring and a predictive approach to support a selection path. The systematic approach may be broadly adapted for other basins. New hydrological insights for the region Understanding regional hydrology hinges on efficient collection of hydrologic data that captures the relevant dynamics including extremes. The present study, a case study for a particular basin in the Southern Rocky Mountains, is the first use of a scripted (R software) strategy to select an economical and representative set of monitoring wells. Our findings suggest caution when using proximity as a proxy for correlation, because proximal wells in the same geologic formation and similar depths are not always correlated. In the Arkansas Headwaters Basin, subsurface geology may be less influential on groundwater elevations than broader hydrologic influences, such as regional drought.

Colorado↗

USGSDate not supplied

Detecting sulfamethoxazole and carbamazepine in groundwater: Is ELISA a reliable screening tool?

In recent years, numerous studies have reported the prevalence of organic micropollutants in natural waters. There is an increasing interest in assessing the occurrence and transport of these contaminants in groundwater because a large number of people in the United States rely on groundwater for their drinking water. However, commonly used mass-spectrometry-based analytical methods are expensive and time-consuming. The enzyme-linked immunosorbent assay (ELISA) method offers an inexpensive analytical alternative that provides semi-quantitative results in a relatively quick timeframe. We investigated the use of ELISA for two commonly detected micropollutants, sulfamethoxazole (SMX) and carbamazepine (CBZ), in groundwater collected as part of two different studies, one in Minnesota and the other in Iowa. The ELISA results were compared with two mass-spectrometry-based methods: (1) direct aqueous injection-high performance liquid chromatography/tandem mass spectrometry (HPLC) and (2) online solid-phase extraction with liquid chromatography/electrospray ionization-mass spectrometry (SPE LC). Differences in SMX and CBZ observations between ELISA and both HPLC and SPE LC were analyzed using the Paired Prentice-Wilcoxon test. Estimates of bias and limits of agreement between paired observations also were calculated. The SMX determinations by ELISA yielded results that were 30 and 14% greater than HPLC and SPE LC, respectively. The CBZ determinations by ELISA yielded results that were 25 and 9% greater than HPLC and SPE LC, respectively. The ELISA determinations were in presence-absence agreement with HPLC for 83% of samples for SMX and CBZ; and with SPE LC for 76 and 80% of samples for SMX and CBZ, respectively. Results indicate that ELISA for SMX and CBZ is a reliable and cost effective screening-tool alternative to more commonly used mass spectrometry-based analytical methods.

Environmental Pollution↗

USGS2025

Statewide cumulative human health risk assessment of inorganics-contaminated groundwater wells, Montana, USA

Across the United States, rural residents rely on unregulated and generally unmonitored private wells for drinking water, which may pose serious health risks due to unrecognized contaminants. We assessed the nature, degree, and spatial distribution of cumulative health risks from inorganic contaminants in groundwater. Our analysis included nearly 84,000 data points from 6500+ wells, across 51 of Montana's 98 watersheds, using a public groundwater database. We compared a drinking water screening level cumulative risk assessment (CRA) for inorganics based on the U.S. Environmental Protection Agency (EPA) protective health thresholds (Maximum Contaminant Level Goals, Health Advisories [MCLG-HAs]) to a CRA based on EPA public supply enforceable Maximum Contaminant Levels (MCLs). Based on median concentrations of 19 inorganics (antimony, arsenic, barium, beryllium, boron, cadmium, chromium, copper, fluoride, manganese, molybdenum, nickel, nitrate, lead, selenium, strontium, thallium, uranium, zinc), 75% of watersheds had MCLG-HA-based cumulative risk values > 1.0; arsenic and uranium contributed the most risk, followed by strontium, fluoride, manganese and boron. Hence, this screening level (Tier I) CRA indicated widespread potential for unrecognized human health risk to private well users from inorganic contaminants considering both carcinogenic and non-carcinogenic risks. Sensitivity analysis showed that benchmarks applied (MCLG-HAs versus MCLs) exerted the largest control on results. Our findings identify priority regions for Tier 2 risk assessments to elucidate local sources and distributions of geogenic versus anthropomorphic contaminants. Our study is the first statewide assessment of cumulative health risk from groundwater that we are aware of, and results support increased statewide drinking water education and testing to reduce human health risks from contaminated private well water.

Montana↗

USGS2017

Highlighting the complexities of a groundwater pilot study during an avian influenza outbreak: Methods, lessons learned, and select contaminant results

The highly pathogenic avian influenza (H5N2) outbreak in the Midwestern United States (US) in 2015 was historic due to the number of birds and poultry operations impacted and the corresponding economic loss to the poultry industry and was the largest animal health emergency in US history. The U.S. Geological Survey (USGS), with the assistance of several state and federal agencies, aided the response to the outbreak by developing a study to determine the extent of virus transport in the environment. The study goals were to: develop the appropriate sampling methods and protocols for measuring avian influenza virus (AIV) in groundwater, provide the first baseline data on AIV and outbreak- and poultry-related contaminant occurrence and movement into groundwater, and document climatological factors that may have affected both survival and transport of AIV to groundwater during the months of the 2015 outbreak. While site selection was expedient, there were often delays in sample response times due to both relationship building between agencies, groups, and producers and logistical time constraints. This study's design and sampling process highlights the unpredictable nature of disease outbreaks and the corresponding difficulty in environmental sampling of such events. The lessons learned, including field protocols and approaches, can be used to improve future research on AIV in the environment.

Iowa↗

USGS2024

Design and calibration of a nitrate decision support tool for groundwater wells in Wisconsin, USA

This paper describes development of a nitrate decision support tool for groundwater wells (GW-NDST) that combines nitrate leaching and groundwater lag-times to compute well concentrations. The GW-NDST uses output from support models that simulate leached nitrate, groundwater age distributions, and nitrate reduction rates. The support models are linked through convolution to simulate nitrate transport to wells. Spatially distributed parameters were adjusted through calibration to 34,255 nitrate sample targets. Prediction uncertainty is illustrated via Monte Carlo realizations informed during calibration. Over 78% of target concentrations were within the simulated range of results from 450 realizations. An example forecasting scenario illustrates that a range of feasible outcomes exist and should be considered when interpreting forecasts for decision making. Uncertainty in forecasting is unavoidable; the intent of characterizing uncertainty in the GW-NDST is to facilitate decision making by increasing insight into the response of nitrate contamination to physical and chemical processes.

Wisconsin↗

USGS2026

Multiple machine-learning estimation of groundwater levels and trends for the regional Mississippi River Valley alluvial aquifer

The Mississippi River Valley alluvial aquifer provides irrigation, public, and domestic water supplies across the south-central United States. Declining groundwater levels require improved characterization of changing conditions. Traditional potentiometric-surface mapping does not use all available water-level data or quantify uncertainty. To address these limitations, we developed a data-driven multiple machine-learning (MML) framework delivered through two open-source R packages. The covMRVAgen1 software assembles covariates to 155,960 monthly groundwater levels from 57,695 wells; the mmlMRVAgen1 software trains Cubist and Random Forest models, blends them, and makes 1-kilometer gridded predictions of monthly potentiometric surfaces for the period January 1980–December 2022. The MML approach provides a methodological foundation for region-scale spatiotemporal groundwater prediction and uncertainty quantification, generating 90-percent prediction limits with appropriate empirical coverage. Model performance is acceptable, with a root-mean-square error of about 4.2 feet, standard deviation of 24.82 feet, and a normalized Nash–Sutcliffe efficiency of 0.973.

Arkansas, Illinois, Louisiana, Mississippi, Missou↗

USGS2020

Sequential biodegradation of 1,2,4-trichlorobenzene at oxic-anoxic groundwater interfaces in model laboratory columns

Halogenated organic solvents such as chlorobenzenes (CBs) are frequent groundwater contaminants due to legacy spills. When contaminated anaerobic groundwater discharges into surface water through wetlands and other transition zones, aeration can occur from various physical and biological processes at shallow depths, resulting in oxic-anoxic interfaces (OAIs). This study investigated the potential for 1,2,4-trichlorobenzene (1,2,4-TCB) biodegradation at OAIs. A novel upflow column system was developed to create stable anaerobic and aerobic zones, simulating a natural groundwater OAI. Two columns containing (1) sand and (2) a mixture of wetland sediment and sand were operated continuously for 295 days with varied doses of 0.14-1.4 mM sodium lactate (NaLac) as a model electron donor. Both column matrices supported anaerobic reductive dechlorination and aerobic degradation of 1,2,4-TCB spatially separated between anaerobic and aerobic zones. Reductive dechlorination produced a mixture of di- and monochlorobenzene daughter products, with estimated zero-order dechlorination rates up to 31.3 µM/hr. Aerobic CB degradation, limited by available dissolved oxygen, occurred for 1,2,4-TCB and all dechlorinated daughter products. Initial reductive dechlorination did not enhance the overall observed extent or rate of subsequent aerobic CB degradation. Increasing NaLac dose increased the extent of reductive dechlorination, but suppressed aerobic CB degradation at 1.4 mM NaLac due to increased oxygen demand. 16S-rRNA sequencing of biofilm microbial communities revealed strong stratification of functional anaerobic and aerobic organisms between redox zones including the sole putative reductive dechlorinator detected in the columns, Dehalobacter. The sediment mixture column supported enhanced reductive dechlorination compared to the sand column at all tested NaLac doses and growth of Dehalobacter populations up to 4.1×108 copies/g (51% relative abundance), highlighting the potential benefit of sediments in reductive dechlorination processes. Results from these model systems suggest both substantial anaerobic and aerobic CB degradation can co-occur along the OAI at contaminated sites where bioavailable electron donors and oxygen are both present.

Journal of Contaminant Hydrology↗

USGS2012

Stochastic analyses to identify wellfield withdrawal effects on surface-water and groundwater in Miami-Dade County, Florida

Several stochastic analyses were conducted in Miami-Dade County, Florida, to evaluate the effects of wellfield withdrawal on aquifer water levels, canal stage, and canal flow. Multiyear data for withdrawals at four water-supply wellfields, water levels at the S-121 canal control structure and groundwater head at a nearby monitoring well were used to determine the interrelation between wellfield withdrawals and water levels in the canal and aquifer. A spectral analysis was performed first on the wellfield withdrawals, showing similar patterns of fluctuations, but no well-defined seasonality. In order to compare water-level response with withdrawals at each wellfield, the intercorrelation effects between wellfields was removed through a ‘causal chain’ approach where the inter-wellfield correlation is used to isolate the wellfield/water-level correlation. Most computed correlations have magnitudes less than 5 percent, but with statistical significance above 90 percent. Results indicate that withdrawals from the wellfields most distant from the canal had no significant correlation to the canal levels. However the highest correlation was not at the wellfield closest to the canal, but at the two wellfields at the intermediate distance that have higher withdrawal rates. The hydraulic interconnectivity of the canal with the rest of the canal network, covering the study area, allows the canal equalizes with all connected canals. This explains why proximity to a particular canal location does not appear to be as important a factor as the withdrawal rate. Groundwater levels are more highly correlated to a wellfield on the same side of the canal, and to pumping wells in the same wellfield on the same side of the canal. This indicates that canals are an effective barrier and source/sink for the groundwater. Further nonlinear correlation analysis indicates that high withdrawal rates disproportionally affect water levels and are the predominant effect on the canal.

Florida↗