Study on turbidity sources for a karst spring downgradient of a marble quarry

Determination of recharge sources for karst springs can be essential in protection against their quantity diminishment and quality deterioration but has been challenged with complexity of karst water systems. This paper discusses an innovative approach consisting of multiple lines of evidence to investigate the water and turbidity sources for a pristine karst spring (Spring #1) that is located adjacent to and downgradient of a marble quarry. The approach is based on data collected on different scales and included characterization of turbidity dynamics in the spring itself, statistical analysis on relationships between turbidity and precipitation as well as between turbidity and blasting, infiltration tests within the quarry pit, X-ray diffraction analysis of sediments samples, and dye tracing tests. Eleven dye tracing tests conducted in five phases confirmed that Spring #1 receives water from both allogenic and autogenic sources. The allogenic recharge is from four sinking streams (Sinking Streams #1 through #4) and sinkholes (Sinkhole #1 area) on the borderlands, whereas the autogenic recharge is from the karst land including the quarry pit. The groundwater flow rates estimated from the tracing tests vary from 9 to 841 m/day, indicative of presence of solution-enlarged conduits in the marble aquifer. The X-ray diffraction analysis of sediment samples collected at Spring #1, the marble quarry, and Sinking Stream #2 suggested multiple turbidity sources in Spring #1. Based on the statistical analysis of data collected over 10 years, precipitation is the most important factor impacting the turbidity in Spring #1. On average, rainfall events can raise the turbidity level by approximately 14 Nephelometric Turbidity Unit (NTU). Although the infiltration test did not identify turbidity changes in Spring #1 in response to pooling of muddy water on the quarry floor, the statistical analysis suggests that, on average, blasting in the quarry could elevate turbidity value by up to 2 NTU. The blasting impact appears to become more noticeable when blasting occurred on the lower two benches of the quarry.