Abstract
Agricultural land in karst systems can pollute water courses, with polluted waters travelling quickly to and through the sub-surface. Understanding how rapidly nitrate moves within the highly-transmissive karst critical zone (from soils to aquifers) is limited by low resolution data. To understand nitrate behavior and its controls, we deployed sensor technology at five sites to generate autonomously high-resolution time series of discharge and NO3−–N, which is the major nitrogenous component, in a farmed karst catchment in Southwestern China. The [NO3−–N] time series exhibited rapid response to rainfall-induced increases in discharge and a large magnitude in [NO3−–N], from 0.72 to 16.3 mg/L across five sites. However, the magnitude of NO3−–N response at each site was varied during rainfall events (wet season) and dry season. The highest mean [NO3−–N] and normalized annual fluvial export occurred in a headwater catchment with a developed karst aquifer system. Seasonal variation in NO3−–N export occurred in response to source availability, most notable in catchments with valley agriculture: in the wet season up to 94% of nitrate was exported from the headwater catchments within two months, but at the larger catchment scale, over the 6 month wet season, only 61% of total export occurred. At the larger catchment scale, [NO3−–N] were lower due to buffering by the karstic aquifer network. From the time series we observe little decrease in [NO3−–N] as discharge decreases in the dry season, indicating the karst aquifers are chronically-polluted with nitrate through slow flow pathways.
Highlights
Nitrate (NO3−) contamination of surface and ground waters is a worldwide concern; it can impact on the ecological quality of aquatic environments and pose a risk to human health if drinking water is contaminated (Van Meter et al, 2017; Zhang et al, 2015)
Where karst hosts agricultural land, evaluating nitrate transfer and attenuation from soil into groundwater i.e. through the critical zone, is a challenge that must be addressed as the following impacts have been observed: i) contaminants can rapidly be transported from surface to groundwater through sinkholes and fracture networks within the karst architecture (Hartmann et al, 2014; McCormack et al, 2016), yet ii) there are areas where contaminants can accumulate and act as a ‘legacy’ source over time (Fenton et al, 2017); iii) the prevalence of rapid transit through the karst aquifer shortens contaminant residence time, reducing capacity for attenuation and affecting receiving water quality (Einsiedl et al, 2009; Katz et al, 2001)
Rainfall was distributed throughout the year with approximately 86% of the annual rainfall 1217 mm (Fig. 2), occurring during the wet season (May to October), in June and August, during November 2016 to October 2017 - which had similar rainfall totals and distribution to the average rainfall in a typical year (1246 mm) (Yue et al, 2018)
Summary
Nitrate (NO3−) contamination of surface and ground waters is a worldwide concern; it can impact on the ecological quality of aquatic environments and pose a risk to human health if drinking water is contaminated (Van Meter et al, 2017; Zhang et al, 2015). Agricultural activities dominate nitrogen (N) delivery to aquatic systems when excessive N fertilizer application exceeds the plant growth requirements (Coxon, 2011; Lassaletta et al, 2014). This scenario is likely in rapid response aquifers, such as those in karst, an important landscape covering approximately 20% of the Earth's ice-free continent area and supplying drinking water for 25% of the world's population (Ford and Williams, 2013; Hartmann et al, 2014). Nitrate loading is a measure of excess nitrogen in the critical zone and should be considered as mass of fluvial nitrate-N (NO3−–N) exported, as this indicates better total NO3−–N losses
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