Search

Without careful land management, agricultural land in karst systems can pollute water courses, with polluted waters travel quickly to and through the sub-surface in karst systems. However, detailed understanding of how rapidly nitrate moves within the karst critical zone (from soils to aquifers) is limited by low resolution sampling for this highly-transmissive system. To understand nitrate behavior and its controlling factors, we deployed sensor technology at five sites to generate autonomously high-resolution time series of discharge and [NO3−N] in a farmed karst catchment in SW China. The [NO3−N] time series exhibited rapid response to rainfall-induced increases in discharge and a large magnitude in [NO3−N], ranging from 0.72 to 16.3 mg/L. The highest mean [NO3−N] and normalized annual fluvial export were observed in one headwater catchment with well-developed conduit structure. Elsewhere this was less-pronounced due to buffering by the karstic aquifer network as the contributing catchment area increased with distance downstream. Clear seasonal variation in NO3−N export occurred in response to source availability, most notable in catchments with valley agriculture: up to 97% of nitrate was exported from the headwater catchment in the wet season, with much of that loss occurring in only a couple of months, compared to 61% of nitrate exported at the larger catchment scale. The sensor time series showed that the aquifers were chronically polluted with nitrate. [ABSTRACT FROM AUTHOR]