TRACING ORGANIC POLLUTANTS IN GRAVEL-PIT LAKES: A CASE STUDY IN THE BRAZOS RIVER ALLUVIUM AQUIFER

Expanding urbanization and growing populations in many areas of the world drives a need for increasing volumes of natural aggregate materials like sand and gravel for construction purposes. In central Texas, this demand often leads to the creation of pit-style mines in alluvial aquifers, where aquifer material is removed, and the pit is left unremediated to create a water feature. These pits serve as direct conduits to aquifer flow and may be potential sources of pollution, particularly when pollutants are dissolved into the water. However, the presence of plants, animals, and oxygenated waters in the lakes themselves may mitigate the introduction of pollutants through uptake and oxidation. There has been little research on the effects of gravel-pit lakes on groundwater chemistry and as both water resources continue to be taxed more heavily and more pits are constructed to keep up with demand for infrastructure development, understanding their effects on the natural system’s ambient chemistry will become increasingly important.

The current project seeks to address the gap in our understanding by investigating two small gravel-pit lakes in the Northern Segment of the Brazos River Alluvium Aquifer near Waco in McLennan County, Texas. Both lakes serve as water features for livestock (cattle) but are dissimilar in terms of their proximity to the Brazos River as well as the lithology of the aquifer material in which they are constructed. Using a Geoprobe 6620 DT, multiple 2-inch piezometers were constructed on both the upgradient and downgradient sides of the lakes, using the river as a guide for the direction of groundwater flow. Over the course of a year, to account for seasonal variation, groundwater samples from these piezometers, and lake samples have been analyzed for dissolved nitrates and phosphates, as well as inorganic and organic carbon. Standard water chemistry and multiple oxygen-18 and deuterium isotope samples were also gathered to provide evidence for the flow through system we hypothesized as well to provide background data. While data gathering is still underway, our preliminary data indicate a flow-through system and the lakes have an effect on the targeted organic pollutants.