Source, variability, and transformation of nitrate in a regional karst aquifer: Edwards aquifer, central Texas

Abstract

Many karst regions are undergoing rapid population growth and expansion of urban land accompanied by increases in wastewater generation and changing patterns of nitrate (NO3(-)) loading to surface and groundwater. We investigate variability and sources of NO3(-) in a regional karst aquifer system, the Edwards aquifer of central Texas. Samples from streams recharging the aquifer, groundwater wells, and springs were collected during 2008-12 from the Barton Springs and San Antonio segments of the Edwards aquifer and analyzed for nitrogen (N) species concentrations and NO3(-) stable isotopes (δ(15)N and δ(18)O). These data were augmented by historical data collected from 1937 to 2007. NO3(-) concentrations and discharge data indicate that short-term variability (days to months) in groundwater NO3(-) concentrations in the Barton Springs segment is controlled by occurrence of individual storms and multi-annual wet-dry cycles, whereas the lack of short-term variability in groundwater in the San Antonio segment indicates the dominance of transport along regional flow paths. In both segments, longer-term increases (years to decades) in NO3(-) concentrations cannot be attributed to hydrologic conditions; rather, isotopic ratios and land-use change indicate that septic systems and land application of treated wastewater might be the source of increased loading of NO3(-). These results highlight the vulnerability of karst aquifers to NO3(-) contamination from urban wastewater. An analysis of N-species loading in recharge and discharge for the Barton Springs segment during 2008-10 indicates an overall mass balance in total N, but recharge contains higher concentrations of organic N and lower concentrations of NO3(-) than does discharge, consistent with nitrification of organic N within the aquifer and consumption of dissolved oxygen. This study demonstrates that subaqueous nitrification of organic N in the aquifer, as opposed to in soils, might be a previously unrecognized source of NO3(-) to karst groundwater or other oxic groundwater systems.

Drivers of Dissolved Organic Matter in the Vent and Major Conduits of the World's Largest Freshwater Spring

“…Karst springs are formed where groundwater flow discharges from a conduit or cave at a vent and becomes surface water. Springs have been observed to typically have high water clarity and stable hydrology, chemistry, and temperature (Berndt et al, ), yet in recent decades, numerous springs in the southern United States of America (USA), including Wakulla Springs, have experienced a change in water color (browning) and shifts in ecology (Arellano & Coble, ; Florida Geological Survey Publication 58, ; Musgrove et al, ). Wakulla Springs is the largest and deepest freshwater spring on Earth and represents a prime example of a site‐facing problems common to many spring systems in the southern United States.…”

Differentiation of Surface Water and Groundwater in a Karst System Using Anthropogenic Signatures

“…While processes within stream benthic sediments could contribute to this disparity between surface/sub-surface water [71][72][73][74], nitrification within the karst aquifer may contribute to the higher values. It was reported by [63] that nitrification was occurring in the Edwards Aquifer but not in the surface streams. While land use in the Edwards Aquifer was predominantly suburban to urban, primary NO 3 -N sources were believed to be fertilizers and human-animal waste.…”

“…Nitrate is a common groundwater pollutant symptomatic of anthropogenic sources, i.e. agricultural, domestic sewages, and animal manure [61,62] and elevated concentrations are common in karst aquifers [5,7,22,63,64]. In karst areas of Illinois, elevated NO 3 -N concentrations have been reported in areas impacted by animal manure or domestic sewage [6,12,16,17,59,65].…”

Recent advances in karst research: from theory to fieldwork and applications

“…Unfortunately, too many of these are contaminated by nitrates and pesticides (e.g. Liu et al 2006;Huebsch et al 2014;Musgrove et al 2016) and even less knowledge is available on pollution by emerging chemicals such as fluorinated substances, medicines (e.g. antibiotics and endocrine disruptors) and micropollutants.…”