CHARACTERIZATION OF VADOSE-ZONE PROPERTIES AND STRATEGY FOR ENHANCING GROUNDWATER RECHARGE IN THE MISSISSIPPI DELTA

The Mississippi River Valley alluvial aquifer (MRVAA) supports an agriculture industry in the Mississippi Delta that produces more than $1.5 billion in annual commodities, but continual decreases in aquifer water levels over in the past few decades suggest that this usage of groundwater is not sustainable. Soil sampling, field measurement of saturated hydraulic conductivity, laboratory analyses to determine water retention curves and saturated hydraulic conductivity values, and model simulations will be conducted to develop a better understanding of recharge to the MRVAA. The soil samples will be collected from two locations near Sky Lake in Belzoni, Mississippi.

While the entirety of the research has not been completed, some preliminary results from data collected at one location can be interpreted. As expected, saturated hydraulic conductivity increases as the grain size of the soil increases with depth. The first ~0.6 m of soil was clay rich, with saturated hydraulic conductivity values ranging from 7x10^–7 to 2x10^–6 m/s, and then the soil transitioned to predominantly silt and eventually sand at a depth of ~3 m, where the saturated hydraulic conductivity was 2x10^–4 m/s.

Once the second group of soil samples are collected in August 2016 and laboratory analysis completed on all samples, data will be correlated and more definitive conclusions can be made. The soil cores will also be visually examined for preferential flow paths; the potential existence of preferential flow on a larger scale will be inferred from comparison of field and laboratory values of hydraulic conductivity. These data will be combined with model simulations to support exploration of vadose-zone wells. Vadose-zone wells are an alternative artificial recharge technique that, unlike surface infiltration methods, are not dependent on permeable surficial soils. Vadose-zone wells are a low cost alternative to traditional saturated-zone wells.

Results of this research thus far are that both the soil grain size and the saturated hydraulic conductivity increase with depth in the Mississippi Delta. A relatively thin layer of clay in the topsoil prevents water from infiltrating the soil and recharging the water table at a desirable rate. Future results will help assess the feasibility of increasing recharge to the MRVAA by using vadose-zone wells.