USING DYE TRACER STUDIES TO ASSESS MUNITIONS CONSTITUENTS TRANSPORT IN KARST TERRANE

Studies at karst sites were used to assess if concentrations of munitions constituents (MC) in surface water or groundwater systems were migrating from firing ranges at concentrations that potentially present an unacceptable risk to human health and the environment.

To assess the groundwater component, conceptual site models were developed (which highlighted the low probability of monitoring wells intersecting major flow conduits in limestone) and hydrologic investigations were planned, which included a dye tracer study. The goal for the studies was to provide a surrogate for monitoring well installations, and to develop an understanding of the direction of karst-groundwater flow, connections to off-site springs, and locations of groundwater divides.

Dye tracer studies were performed over an approximate sixteen-week period and included background monitoring rounds and up to ten sampling rounds after dye placement or injection. Commonly used dyes for these assessments included Rhodamine-WT, fluorescein, and eosine. There were up to 700 charcoal samples and up to 200 water samples collected during a typical study.

The studies resulted in finding hydraulic connections between placement areas and local or regional springs/streams. In addition, dye detections were monitored in the downstream direction to assess any additional or incremental dye inputs along the flow path. Straight-line travel rates varied from hundreds of feet per day to >6 miles per day. There were varying levels of confidence in a dye detection that ranged from high (for mass detections in regional springs) to low (for sporadic and low-level detections, typically in monitoring wells).

The dye tracer studies were mostly done during high flows in the wet season. Some variation in flow is expected during times of less intense precipitation and fluctuating recharge during the water year. Additional dry season studies would bolster these results.

After the assessment of hydraulic connections between source areas and springs, groundwater samples were collected to assess MC concentrations in the spring discharges. Samples were collected during clear weather and storm events. The storm samples were timed for collection during peak storm flow from the karst, which was normally several hours after peak flow in the abutting surface water.