Grantee Research Project Results
Using Stable Isotopes to Quantify Sources of Water and Characterize Nutrient Fluxes in an Ozark Cave Stream, ArkansasEPA Grant Number: FP917347
Title: Using Stable Isotopes to Quantify Sources of Water and Characterize Nutrient Fluxes in an Ozark Cave Stream, Arkansas
Investigators: Knierim, Katherine J
Institution: University of Arkansas - Little Rock
EPA Project Officer: Packard, Benjamin H
Project Period: August 1, 2011 through July 31, 2014
Project Amount: $126,000
RFA: STAR Graduate Fellowships (2011) RFA Text | Recipients Lists
Research Category: Fellowship - Water Quality: Hydrogeology and Surface Water , Academic Fellowships
This research aims to quantify water sources in a cave stream during base flow and storm events using stable isotopes to better assess how contaminants – including nutrients, bacteria and sediment – impact cave and spring water quality. Much research has focused on water quality at springs, but sampling within a cave conduit provides access to groundwater in equilibrium with gaseous voids and the ability to directly sample bedrock matrix waters. Quantifying the correlation between water sources and water quality in caves will identify the source areas and mechanisms of contaminant transport through heterogeneous karst systems, which is important for developing management strategies to protect karst aquifers from contamination.
Isotopic hydrograph separation techniques will be used to quantify source water contributions in a cave stream and relate these water sources to water quality. A three-component hydrograph separation will be completed using stable isotope ratios of water (δD and δ18O) and the concentration and isotopic composition of dissolved inorganic carbon (δ13C) to quantify proportions of precipitation, soil and bedrock matrix water in a cave stream during storm events. The detailed hydrologic budget for recharge developed by quantifying water sources in the cave stream allows contaminant flux to be related to the proportions of diffuse (soil and bedrock matrix water) versus quick flow (precipitation). Additionally, dissolved organic carbon and nitrogen species will be analyzed for isotopic composition to better assess carbon and nitrogen sources and biogeochemical cycling along groundwater flow paths. Seasonal influences will be addressed by monitoring storms following wet and dry antecedent conditions during the spring and fall, respectively.
Developing a hydrologic budget using isotopic hydrograph separation techniques will quantify groundwater versus surface water sources to karst springs and highlight variability within and between storm events. Hydrograph separation techniques in karst settings have focused on storm-flow hydrographs from springs, but the geochemistry of spring waters that have encountered air-filled voids may be different than those moving through saturated conduits and fractures. Therefore, this research will explore the interaction of the cave stream with the cave atmosphere—a potentially important control on water geochemistry because of seasonal variations in carbon dioxide concentration in the cave atmosphere. Source water contribution (diffuse versus quick flow) and the partitioning of contaminants between dissolved and particulate phases are expected to control the flux of contaminants during storm events.
Potential to Further Environmental / Human Health Protection
In many cave and spring systems, an inability to identify sources of contaminants continues to hinder the development of best management practices for reducing non-point source pollution. Water quality impacts in northwestern Arkansas due to suburban and urban development and agriculture will be addressed to characterize contaminant transport along sensitive cave flow paths. These research findings will have broad application to karst systems around the world, which is of particular importance because karst aquifers are a global water resource and are experiencing increasing stress due to population growth in karst regions.