LONG-TERM MONITORING OF WATER LEVELS AND CHEMISTRY IN GROUNDWATER OF THE ARKANSAS AREA, MISSISSIPPI EMBAYMENT, USA: PRELIMINARY RESULTS

The Mississippi Embayment, located in the southeastern U.S., is a syncline formed by the northward excursion of the Gulf of Coastal Plain. Structurally, the Mississippi Embayment is a hydrogeological basin consisting of six regional aquifers. These productive aquifers yield good-quality waters.

Across state lines, consistent names for the hydrogeologic units were used. The uppermost unconfined horizon forms the Mississippi River Valley Alluvial Aquifer (ALVM), one of the largest unconfined aquifers in the world. The Holocene and Pleistocene ALVM is formed from sand, gravel, and loess. The majority of the groundwater wells is drilled in the ALVM. As the groundwater levels have fallen in the unconfined ALVM, more groundwater wells are drilled in the deeper aquifers—the Upper, Middle, and Lower Claiborne Aquifers. The Ecocene Upper Claiborne Aquifer protolith is sand, silt, and clay while the Ecocene Middle Claiborne and Lower Claiborne aquifers are sand and minor clay.

We focused our investigation of the spatial and temporal evolution of groundwater in the Arkansas section of the Mississippi Embayment by using wells with long term monitoring records, some of them reaching back to the 1950's. Overall, the groundwater levels of the unconfined aquifer (ALVM) have decreased; we have not yet evaluated the lower aquifers. Attention was paid to rock-water interactions along flowpaths in all aquifers, and to temporal changes at specific sampling sites. The study is based on data from the USGS groundwater monitoring database, utilizing groundwater cation, anion, and nutrient data in the programs AquaChem and PHREEQE.