Using Isotopes to Assess Transit Times and Pollution Risk in Mesoscale Catchments

The use of environmental tracers to discern dominant runoff sources has become an increasingly popular approach in the field of hydrology. Developing an understanding of both geographical water sources and the timing of water passage through a catchment provides an avenue to characterise both the spatial and temporal dynamics of water fluxes in rainfall-runoff transformation. Understanding these dynamics can provide invaluable insight into the the nature of pollution risk and longevity of natural clean-up times in different catchment landscapes. Here, we present results from a study investigating catchment behaviour across eight heterogeneous mesoscale (104-488 km2) catchments in the north east of Scotland. Weekly samples were taken of both streamwater and precipitation for stable isotopes (2H and 18O) to facilitate Mean Transit Time (MTT) estimates at the catchment scale. Streamwater samples were also analysed for alkalinity which was used as a hydrochemical tracer to determine runoff sources. Estimates of MTT were conducted using a gamma distribution convolution integral model and controlling catchment characteristics were identified using multiple regression using a GIS of landscape properties. A range of MTTs were estimated for the study catchments. In more upland catchments these were in the order of 2 years as waters were mainly derived from near surface soil horizons, but in more lowland catchments the transit times increased to over 4 years as groundwater became a more significant contributor to flow. In such lowland catchments, a legacy of fertilizer applications has contaminated groundwater sources which impacts on macronutrient levels in stream waters. Clean up times are likely to be decadal, and meantime short residence time, clean upland waters provide a critical ecosystem service in diluting downstream sources and maintaining stream water quality.