Fire retardant and post-fire nutrient mobility in a mountain surface water—karst groundwater system: the Hidden Fire, Sequoia National Park, California, USA

Abstract

Nutrient mobility in strongly coupled surface water–groundwater systems is not well studied in fire dominated ecosystems. In 2008, the Hidden Fire in the Kaweah River basin in the Sierra Nevada, CA, USA provided an unexpected opportunity to document how nutrient concentrations change post-fire in a karstic groundwater—surface water system. The results from 4 years of sampling and water quality data (2009–2012) suggest that nutrient byproducts from 94 Phos-Check D75 R fire retardant that was dropped to combat the fire were mobilized into the aquatic system. Dissolved nitrate concentrations sharply increased at most monitoring sites with the onset of winter precipitation and seasonal snowmelt in the spring of 2009, remained elevated during seasonal sampling at some sites in 2010, and were significantly lower in 2011 and 2012. Average nitrate concentrations in sub-basins during the 2009 high-discharge period are strongly correlated with the amount of retardant applied in each sub-basin, but not correlated with the sub-basin area. Dissolved phosphate concentrations were highest upstream of sink points where streams directly recharge marble karst aquifers, and decreased between recharge sites and springs; indicating organic and/or inorganic uptake of P in the subterranean system. Although measured nutrient concentrations were within the range documented post-fire in other aquatic systems that did not receive fire retardant, evidence from the Hidden Fire strongly suggests that fire retardant derived nutrients entered and flushed through the Yucca Creek system, and that managers should consider the potential impacts of retardant application on aquatic ecosystems even when retardant is not applied directly on waterways .