Mississippi River Alluvial Aquifer Research Articles

Unstructured-grid approach to develop high-fidelity groundwater model to understand groundwater flow and storage responses to excessive groundwater withdrawals in the Southern Hills aquifer system in southeastern Louisiana (USA)

Study regionThe Southern Hills aquifer system (SHAS) in the Louisiana Capital Area Groundwater Conservation District (CAGCD), USA. Study focusThe SHAS provides abundant groundwater for public and industrial supplies in the CAGCD. Groundwater depletion, saltwater intrusion, and land subsidence are potential concerns due to prolonged excessive groundwater withdrawals. This study develops a high-fidelity groundwater flow model utilizing a complex unstructured grid to investigate groundwater flow and storage responses to excessive groundwater withdrawals for the SHAS in the CAGCD. The groundwater model incorporates the Mississippi River alluvial aquifer down to the Miocene sands extending to depths around 1 km. New hydrological insightsGroundwater modeling results indicate large cones of depression in the Evangeline and Jasper formations in the Baton Rouge area due to prolonged groundwater withdrawals. Low-permeability faults are inferred by significant groundwater level difference across the faults. While local groundwater storage depletion in deeper aquifers is evident, overall estimated groundwater storage changes of the SHAS in the CAGCD are close to zero in the past two decades, indicating insignificant groundwater storage changes. This is attributed to dominant interactions between the major rivers and the shallower alluvial aquifer. In addition, the simulated groundwater storage changes exhibit patterns similar to those derived by the Gravity Recovery and Climate Experiment (GRACE) model that has been used in evaluation of groundwater depletion in many regional studies.

Potential economic impacts of groundwater conservation in the Mississippi River Alluvial Aquifer (MRAA), Louisiana, USA

AbstractOverextraction of groundwater reduces groundwater height, increases the energy cost, and may threaten an aquifer's economic life. Water‐intensive crops, corn, and soybean, dominate the agricultural land in the Mississippi River Alluvial Aquifer (MRAA) region of the United States, thus stressing this confined aquifer. Groundwater conservation policy or the adoption of efficient irrigation technology could save both water and energy. This study aims to estimate the future returns from the irrigated land under the scenarios of 30%, 20%, 10%, 5%, and no groundwater conservation from 2020 to 2022. An accurate model to predict the crop choice decision is important to estimate the impact of groundwater policies. We develop a crop choice model where an individual farmer has a crop planting or land fallowing choice each year. We use the random forest, boosted regression trees, and support vector machine for the crop choice prediction. Boosted regression trees perform the best in our classification problem with 75.5% out of sample accuracy. The prediction model shows that the numbers of corn growers increase in the future. Our results show that the profit of 2572 farmers increased cumulatively by 0.14% when they conserve groundwater by 30% for 3 years. From a policy perspective, providing financial and technical assistant to farmers for making investments to conserve groundwater could save energy costs and sustain the economic life of the MRAA.Recommendations for Resource Managers Land allocations to water‐intensive crops such as corn and soybeans will expand in Louisiana. More efficient irrigation technology may reduce the groundwater extraction volume. Groundwater extraction volume using a choice prediction model gives a more accurate account of water use compared to the status quo crop choice decision scenario. Since efficient irrigation measures do not reduce crop yield, profit from the irrigated land is higher as it reduces the extraction cost. Government cost‐share to adopt efficient irrigation measures may help conserve groundwater resources.

Accounting for uncertainty in complex alluvial aquifer modeling by Bayesian multi-model approach

Alluvial aquifers by nature are complex caused by varied depositional environments. Developing a reliable groundwater model to represent an alluvial aquifer is non-trivial. Also, relying on a single best calibrated model may not be sufficient because of an inadequate choice of model parameter values. To better understand groundwater dynamics and improve model prediction reliability, this study presents a Bayesian multi-model uncertainty quantification (BMMUQ) framework to account for model parameter uncertainty in complex alluvial groundwater modeling. The methodology was applied to the agriculturally intensive Mississippi River alluvial aquifer (MRAA), Northeast Louisiana. An aquifer architecture was first constructed using 7,259 well logs in the MRAA area which covers three fluvial deposits (alluvium, braided-stream terrace, and braided-stream terrace-loess). A 12-layer MODFLOW model was then developed to address the alluvial aquifer complexity and well calibrated through a genetic algorithm. This study quantified model parameter uncertainty in hydraulic conductivity and specific storage of sand facies. Bayesian model averaging (BMA) with the Expectation Maximization (EM) algorithm was adopted to derive posterior model weights and head variances of 50 alternative conceptual groundwater flow models, and thereby obtains BMA ensemble model predictions instead of only relying on the best calibrated conceptual model. Results show that an estimated around 950 million m3 of groundwater storage loss occurs in 2015 with respect to the beginning of 2004, due to high groundwater demand for irrigation in the MRAA area. Explicitly quantifying model uncertainty can produce more reliable groundwater level predictions from BMA ensemble model. The presented groundwater modeling framework improves our understanding of the MRAA and provides a valuable tool to assist agricultural water management.

Pleistocene sands of the Mississippi River Alluvial Aquifer produce the highest groundwater arsenic concentrations in southern Louisiana, USA

Geostatistical and geospatial analysis techniques were employed to evaluate relationships between high arsenic (As), iron (Fe), and manganese (Mn) concentrations in groundwaters from the Pleistocene Mississippi River Alluvial Aquifer (MRAA) in south-central Louisiana. The MRAA is recharged by the Mississippi River, and its groundwater contains Fe concentrations that range from 0.43 to 179 µmol kg−1 (n = 40) and As concentrations that range from 26 nmol kg−1 up to 8500 nmol kg−1 (i.e., 637 µg kg−1). Arsenic speciation analysis reveals a mix of redox states with As(V) accounting for between ca. 18% and ca. 84% of the total dissolved As (mean ± 1σ = 61.6 ± 24.4%, n = 6). Spatial distributions of As, Fe, and Mn concentrations suggest that the reductive dissolution of Fe/Mn oxides/oxyhydroxides is the likely source of As to the MRAA groundwaters. Geostatistical and geospatial analyses were employed to produce interpolation maps that show concentrations of As, Fe, and Mn decrease with distance from the Mississippi River. Spatial correlations are stronger between groundwater As and Fe concentrations (r = 0.710, PAs*Fe < 0.0005) than those between the As and Mn concentrations (r = 0.404, PAs*Mn < 0.0005). More specifically, regions of the MRAA characterized by high dissolved As concentrations spatially correlate with regions of high dissolved Fe concentrations, and to a lesser degree, high dissolved Mn concentrations. We hypothesize that recharge of the MRAA by Mississippi River water introduces labile organic carbon into the MRAA that subsequently fuels microbial respiration, which reduces Fe/Mn oxides/oxyhydroxides, and mobilizes adsorbed and/or co-precipitated As into MRAA groundwaters. Once mobilized, As is transported away from the riverbank by flowing groundwaters but is rapidly removed from solution by re-adsorption onto Fe/Mn oxides/oxyhydroxides within the regionally oxidized aquifer sediments. Our investigation indicates that oxidized Pleistocene sand aquifers in large river delta plains can also produce groundwater with dangerously high As concentrations if redox conditions shift to suboxic/ferruginous conditions.

Lake County, Tennessee, in the Heart of the New Madrid Seismic Zone

Lake County, Tennessee, lies within the center of the New Madrid seismic zone and thus is particularly vulnerable to seismic hazards. To better evaluate the seismic threat to Lake County, the stratigraphy and structure were mapped beneath the county. A shallow 3-D lithologic model was constructed to illustrate the Quaternary Mississippi River alluvial facies of Lake County in five-foot (1.5 m) thick layers to a depth of 300 feet (91.4 m). This model provides near-surface geologic data for the determination of earthquake liquefaction potential and illustrates the Mississippi River alluvial aquifer for optimum agricultural water exploitation. In addition, faulted structure contour maps were made of the tops of the unconformable Paleozoic, Cretaceous, and Eocene sections. With these structure contour maps a deep 3-D geologic model was built for Lake County, which shows the stratigraphy and structure from the Quaternary to the Paleozoic. Cross sections were also created to illustrate the subsurface geology. The faulted structure contour maps and cross sections reveal Quaternary faulting on the Reelfoot North, Axial, Tiptonville dome backthrust, Lake County Uplift backthrust, Cottonwood Grove, Ridgely, and previously unidentified faults. The maps also illustrate where surface fault rupture may occur and the sense of vertical surface displacement in the event of a future large earthquake.

The origins of high concentrations of iron, sodium, bicarbonate, and arsenic in the Lower Mississippi River Alluvial Aquifer

Many alluvial aquifer systems are characterized in part by groundwater rich in sodium (Na) and bicarbonate (HCO3) and elevated concentrations of dissolved iron (Fe) and arsenic (As). However the relationships among Fe and As concentrations and the formation of NaHCO3 waters are unclear. Here we evaluated the formation of NaHCO3 groundwater in the Lower Mississippi River Alluvial Aquifer (LMRAA) in Louisiana, USA, where concentrations of dissolved Fe and As reached values as high as 16 mg/L and 64 μg/L, respectively. Water isotope data showed that groundwater was predominantly recharged via local precipitation outboard of the Mississippi River, while groundwater nearer the river and to the south of the study area was more hydraulically connected to the river. Much of the geochemical data, including water isotopes, calcite saturation index, Na excess, and As correlated to some degree with well depth, highlighting the complex interactions between the shallower recharge from local precipitation and the deeper reaction pathways laterally connected to the Mississippi River and/or vertically connected with regional groundwater. The δ13C(DIC) of groundwater ranged from −17.4 to −11.1‰, likely representing a mixture of microbially- and abiotically-derived alkalinity. Within this context, the production of groundwater rich in NaHCO3 can be explained by reaction pathways involving the weathering of CaNa silicate phases. This weathering, driven in part by the production of microbial CO2 during Fe and sulfate reduction, may continue past the point of calcite saturation. Once calcite reaches saturation the concentration of Ca is buffered, but there is a possibility of further buildup in the concentrations of Na and HCO3. The silicate weathering model serves as an alternative conceptual model to calcite dissolution and clay exchange for the development of NaHCO3 waters. If globally important, this water-rock reaction pathway could place new constraints on the processes influencing the fate and transport of Fe and As in alluvial aquifer systems.

Identification of recharge zones in the Lower Mississippi River alluvial aquifer using high-resolution precipitation estimates

Summary Water resources in the lower Mississippi River alluvial valley play a critical role in agricultural productivity due to the widespread use of irrigation during the growing season. However, the unknown specifics of surface–atmosphere feedbacks in the region, along with diminishing groundwater availability and the non-sustainable trend in irrigation draws from the alluvial aquifer, makes it difficult for water resource managers to make sound decisions for future water sustainability. As a result, it is crucial to identify spatial and temporal associations between local rainfall patterns and groundwater levels to determine the influence of precipitation on regional aquifer recharge. Specifically, it is critical to define the recharge zones of the aquifer so that rainfall distribution can be used to assess potential groundwater recovery. This project addresses the issue of defining areas of recharge in the lower Mississippi River alluvial aquifer (LMRAA) through an assessment of historical precipitation variability using high-resolution radar-derived precipitation estimates. A rotated principal component analysis (RPCA) of both groundwater and precipitation data from October through April is used to define locations where aquifer levels show the greatest variability, with a stepwise regression approach used to define areas where rainfall and groundwater levels show the strongest association. Results show that the greatest recharge through direct rainfall is along the Tallahatchie River basin in the northeastern Mississippi Delta, with recharge along the periphery of the LMRAA likely a result of direct water flux from surface hydrologic features.

Sources of elevated salinity in the Mississippi River Alluvial Aquifer, south-central Louisiana, USA

Abstract Salinization is a process impacting groundwater quality and availability across much of southern Louisiana, USA. However, a broad divergence of opinion exists regarding the causes of this elevated salinity: updip-migrating marine waters from the Gulf of Mexico, saline fluids migrating up fault planes, movement of water from salt domes, and/or remnant seawater from the last major marine transgression. The Mississippi River Alluvial Aquifer (MRAA) in south-central Louisiana is recharged by the Mississippi River, and there are discharge zones to the west and east. Recharge waters from the Mississippi River are fresh, but Cl− levels in the western portions of the aquifer are as high as 1000 mg/L. The aquifer is an important source of water for several municipalities and industries, but prior to this study the source(s) of the elevated salinity or whether the salinization can be remediated had not been determined. The low Br/Cl ratios in the groundwaters are consistent with a saline endmember produced by subsurface dissolution of salt domes, not a marine source. The H and O isotopic systematics of the aquifer waters indicate meteoric sources for the H2O, not marine waters or diagenetically-altered deep brines. The westward salinization of aquifer water represents a broad regional process, instead of contamination by point sources. Mapping of spatial variations in salinity has permitted the identification of specific salt domes whose subsurface dissolution is producing waters of elevated salinity in the aquifer. These include the Bayou Choctaw and St. Gabriel domes, and possibly the Bayou Blue dome. Salinization is a natural, on-going process, and the potential for remediation or control is slight, if not non-existent.

Agricultural Chemicals in the Alluvial Aquifer of a Typical County of the Arkansas Delta

Statistical methods and a Geographic Information System (GIS) were used to investigate potential indicators of ground water vulnerability to agricultural chemical contamination in a representative area of the Mississippi River alluvial aquifer. A total of 47 wells were sampled for analysis of nitrate, phosphorus, potassium, and 13 pesticides commonly-used in the area. Ten soil and hydrogeologic variables and five ground water vulnerability indices were examined to explain the variations of chemical concentrations. The results showed that no individual soil or hydrogeologic variables or their linear combinations could explain more than 25% of the variation of the chemical concentrations. A quadratic response surface model with the values of confining unit thickness, slope, soil permeability, depth to ground water, and recharge rate accounted for 62% of the variation of nitrate, 43% of P, and 83% of K, suggesting that the interactions among soil and hydrogeologic variables were significant. Observed trends of decreasing nitrate and P concentrations with increasing well depth and/or depth to ground water seemed to correlate with carbonate equilibrium in the aquifer and more reduced environment with depth. In view of uncertainties involved, it was recognized that the limitations associated with input data resolution used in GIS and the formulation of leaching indices limited their use for predicting ground water vulnerability. Misuse of pesticides could be another factor that would complicate the relationships between pesticide concentrations and the vulnerability indices.

Acute and chronic toxicity of the herbicide Stam M-4 in field and laboratory exposures.

Propanil (3',4'-dichloropropionanilide), the active ingredient in the herbicide Stam M-4, is possibly the most extensively used herbicide for rice production in the world. Propanil and its metabolites are transported within characteristic ditch ecosystems in the production landscape of northeast Arkansas. Runoff from these ditch ecosystems is further transported to a river or other water body supplied by the Mississippi River Alluvial aquifer. Forty-eight-hour acute toxicity tests with Ceriodaphnia dubia (cladoceran) and Pimphales promelas (fathead minnow) were conducted on stormwater runoff, laboratory synthetic water, and irrigation (ground) water. No effects on survival were observed in this study following 48-h toxicity testing with the stormwater. Survival studies indicated assimilative capacity in irrigation (ground) water as opposed to laboratory synthetic water. Mean 48-h LC50s of C. dubia increased from 2.94 mg/L Stam M-4 in laboratory synthetic water to 8.01 mg/L Stam M-4 in irrigation water. Likewise, P. promelas mean 48-h LC50s increased from 23.76 (laboratory synthetic water) to 33.52 mg/L Stam(R)M-4 (ground water). In 7-d chronic tests, there was an increase in mean LC50s of C. dubia when comparing synthetic water to irrigation water (0.48 to 1.24 mg/L Stam M-4, respectively). P. promelas, however, had less tolerance for Stam M-4 in irrigation water (4.45 mg/L) than in synthetic water (5.93 mg/L) in 7-d chronic toxicity tests. Forty-eight-hour toxicity tests indicate that ground water affords organisms some assimilative capacity that laboratory synthetic water does not. Since herbicides and most other pesticides are manufactured to elicit rapid responses, 48-h toxicity results best describe potential nontarget organism effects in aquatic ecosystems.

Depositional and pedogenic influences on the environmental geology of Holocene Mississippi River floodplain deposits near Ferriday, Louisiana

Core descriptions and the mineralogy of Holocene Mississippi River floodplain deposits in Louisiana provide insights on fluid migration pathways and the origin of iron-rich ground waters in the Mississippi River Alluvial Aquifer (MRAA). Vertical changes in the sedimentologic and pedologic characteristics of floodplain deposits near Ferriday, Louisiana, provide evidence for two stages of floodplain development and suggest that depositional processes and drainage conditions changed substantially during the Holocene. Depositional and pedogenic processes produced complex fluid migration pathways in the MRAA confining unit and also contributed to the formation of iron-rich ground waters. Lower Holocene deposits in the study area are older than ∼5000 yrs BP and show evidence of crevassing, lacustrine delta building, and multi-channel stream deposition. These processes deposited thin and narrow sheet sands, which represent fluid migration pathways in the MRAA confining unit. Poor drainage conditions during this initial stage of floodplain development also favored the precipitation of authigenic siderite and pyrite in poorly-drained swamps and shallow lakes. The pyrite and siderite probably represent the source of iron-rich ground waters in the MRAA. Upper Holocene floodplain deposits are younger than ∼ 5000 yrs BP and represent the transition to the present-day meandering regime of the Mississippi River near Ferriday. This second stage of floodplain development was accompanied by pedogenesis, which produced slickensides in clayey backswamp soils. The abundance of slickensides and the presence of the thin and narrow sheet sands indicates that fluid migration in the MRAA confining unit near Ferriday is greater than generally recognized. Seasonal water table fluctuations and the mixing of oxygenated meteoric and reduced ground waters cause iron oxide reduction and pyrite oxidation in backswamp settings, which releases iron into solution. The presence of high (up to 16 mg/l) dissolved iron concentrations in water wells that are screened beneath pyrite- and siderite-rich, muddy backswamp deposits near Ferriday, suggests that the distribution of fine-grained alluvium is a primary control on the presence of iron-rich waters in the MRAA. In contrast, water wells that are screened in sandy meander belt deposits, which lack abundant iron-bearing minerals, have low (less than 1 mg/l) concentrations of total dissolved iron. Studies of large floodplains such as the Mississippi River, highlight the importance of floodplain histories for evaluating geologic influences on water quality, developing proper floodplain land use strategies, and for improving our understanding of the environmental geology of floodplain systems.

Geohydrologic Controls on the Distribution and Origin of Brackish Water in the Mississippi River Alluvial Aquifer of Eastern Morehouse Parish, Louisiana, and Southern Chicot County, Arkansas: ABSTRACT

Geohydrologic Controls on the Distribution and Origin of Brackish Water in the Mississippi River Alluvial Aquifer of Eastern Morehouse Parish, Louisiana, and Southern Chicot County, Arkansas: ABSTRACT