GROUNDWATER QUALITY RESPONSE TO FLOODING FOLLOWING HURRICANE HANNA CONSTRAINED BY GEOCHEMICAL AND BIOLOGICAL INDICATORS

Hurricane Hanna (HH) made landfall on July 25, 2020, along the south Texas coast, motivating this study. HH impacted coastal, rural, and urban areas with precipitation between 30-45cm, storm surges over 60cm high in Baffin Bay, and floodwaters in the Rio Grande Valley (RGV) over 1m deep. Floodwaters can mobilize and transport anthropogenic contaminants, such as fertilizers, pesticides, hydrocarbons, fecal indicator, and pathogens, to rivers, streams, and groundwater. On a broader scale, these contaminants, if left untreated can cause serious illness if ingested by humans. This project aimed to determine the aquifer’s resilience to flooding from HH in inland, urban and rural settings via biological and geochemical evaluations. As anthropogenic activities increase, it is vital to understand how surface- and groundwater respond and recover from storm events. The data shows that bacteria and other contaminants entered the groundwater both inland and along the coast, due to surface runoff and/or storm surge. Notable levels of E. coli were recorded in surface water along the coast immediately following HH landfall, likely due to septic tanks overflowing and/or animal waste runoff. Throughout the project E. coli was generally much lower in urban areas than in rural areas in the RGV area. Nitrate, on the other hand, responded differently, depending upon flooding status: Samples from the flooded areas had an initial increase in nitrate concentration, while samples in non-flooded areas that did not showed a decrease in nitrate. After a month, all samples in the RGV follow a similar trend in nitrate concentration. Additionally, sampled wells located in areas that flooded had the lowest salinities immediately after landfall and showed an upward trend over time after floodwaters retreated, likely indicating that recharge from floodwaters was rapid, and entered the aquifer either directly through the well head or infiltrated along failing well casings. In contrast, areas not impacted by floodwaters initially had higher salinity compared to one month after HH, when the salinity dropped before gradually increasing in the following months. Further data analyses are conducted to evaluate how seasonal changes in hydroclimatic conditions impact groundwater biogeochemistry.