Coupling Field and Laboratory Experimental Data to Better Characterize Non-Conservative Tracing Responses

Groundwater tracing is increasingly used in hydrogeology for quantitative objectives, such as the calibration of flow and transport models. Confident application places high demands on tracers employed to avoid interpretive shortcomings while analyzing breakthrough curves obtained. Most notably, those tracers assumed to reflect purely advective and dispersive processes should not react with aquifer materials.

Many dye tracers are widely used in investigations in karst systems where interactions with conduit walls are assumed insignificant. However, field-based comparative tracing experiments suggest that several of them may display non-conservative behavior. A better understanding of field-based tracer interactions with aquifer materials requires characterization under more controlled conditions.

Laboratory experiments exposed a range of solute and particle tracers to karst-related microcosms. The tracers showed variable levels of reaction, depending on tracer type, substratum and hydrochemistry. This permitted the processes, by which different tracers are potentially affected by interactions with conduit surfaces, such as sorption and biodegradation to be better characterized. Coupling these data with field-scale tracing responses permits more confident interpretation of breakthrough curves. Furthermore, once tracer-specific properties are known in detail, one can start to employ them more confidently as reactive surrogates to better understand contaminant fate and transport in karst environments.

The findings of this study highlight the potential for tracers to be attenuated by reactions with conduit surfaces and the possibility that many dye tracers may not be unreactive under these conditions, as previously assumed. Conversely, improved characterization of these substances may permit them to be used to provide valuable information on contaminant-specific transport through karst. As a corollary to this, the differential responses of different dyes during tracer testing may be used to more confidently make inferences about subsurface geochemical conditions, based on prior behavior in microcosms.