Identification and Prioritisation of Mine Pollution Sources in a Temperate Watershed Using Tracer Injection and Synoptic Sampling

Abstract

Identifying and prioritising mine sites for remediation is challenging due to inherently complex hydrological systems and multiple potential sources of mine pollution dispersed across watersheds. Understanding mine pollution dynamics in wet temperate watersheds is particularly challenging due to substantial variability in precipitation and streamflows, which increase the importance of diffuse sources. A tracer dilution and synoptic sampling experiment was conducted in a mined watershed in Scotland to identify the main sources of mine pollution, the relative importance of point and diffuse sources of pollution, and the potential benefits of mine site remediation to stream water quality. Using high spatial resolution metal loading datasets, the major Zn and Cd source areas were identified as point sources of mine water predominantly located in the upper part of the watershed. In contrast, the main sources of Pb were diffuse sources of mine tailings and wastes located in the lower part of the watershed. In the latter case, mobilisation of Pb occurred primarily from a section of braided wetland and an uncapped tailings area. Importantly, diffuse sources of mine pollution were found to be the dominant source of Pb, and an important source of Zn and Cd, even under steady-state streamflow conditions. Mass balance calculations suggest that treatment of the main mine water sources in the upper watershed and capping of the tailings deposit in the lower watershed could reduce stream trace metal concentrations by approximately 70%. These data support the development of conceptual models of mine pollution dynamics in wet temperate watersheds. These conceptual models are important as they: (1) help prioritise those mine sites and features for remediation that will deliver the most environmental and socio-economic benefit, and; (2) provide a means to quantify the importance of diffuse pollution sources that may increase in importance in the future as a result of changes in precipitation patterns in temperate watersheds.

Identification and quantification of diffuse groundwater pollution in a mineralised watershed

“…Due to the high spatial resolution stream metal loads possible with the synoptic sampling and tracer injection approach, areas of diffuse groundwater pollution otherwise undetected can be located and quantified, and then considered as part of a whole site remediation plan. 11 Tracer injection and synoptic sampling…”

“…velocity-area, gulp injection]. 11,30 First, as the tracer is injected over a period of days it becomes fully mixed in the stream and therefore captures flow in irregular channel sections and the hyporheic zone. Secondly, as the streamflow is calculated using the tracer concentration in the stream, sampling can be carried out rapidly at each sample site, maximising the number of samples that can be taken and increasing the spatial resolution of sampling.…”

“…Comparing the cumulative instream load to the metal load calculated for the individual or grouped stream segments, highlights locations in the stream where metal attenuation (through chemical reactions) or metal addition (through sources) are occuring. 11 Furthermore, the percent contribution of each source can be calculated as:…”

“…The tracer dilution and synoptic sampling technique, 10 originally developed by the U.S. Geological Survey, has the potential to be used for mine impacted groundwater source apportionment in the more temperate climates of the UK. 11,12 The tracer dilution technique is used to calculate streamflow (Q) from the dilution of an injected conservative tracer. As Q values are derived from the downstream changes in tracer concentration, and the tracer has been thoroughly mixed into the river over several days, these flow values are not affected by irregular stream cross-sections, as the velocity-area method would be, and include flow in the hyporheic zone, unlike a weir, or the gulp-injection method.…”

“…As Q values are derived from the downstream changes in tracer concentration, and the tracer has been thoroughly mixed into the river over several days, these flow values are not affected by irregular stream cross-sections, as the velocity-area method would be, and include flow in the hyporheic zone, unlike a weir, or the gulp-injection method. 11 Combined with pollutant concentration data collected at the same time and location as the flow data, this method produces high spatial resolution profiles of pollutant loads that can be used to apportion sources of pollution.…”