Theme: 05. Fractured Rock and Complex Geology

Topic Area: 5c. Remediation Approaches in Fractured Rock and Karst Aquifers

Formatted Title:

Hydraulic Response to Periodic Purging of DNAPL from a Fractured, Karstified Limestone Aquifer

Background/Objectives:

The subject site is located in the Western Pennyroyal karst area of Kentucky. A perimeter well installed in the Ste. Genevieve limestone to help delineate the extent of tetrachloroethene in groundwater was determined to contain up to 17 feet of dense non-aqueous phase liquid (DNAPL) after initially yielding no appreciable groundwater. Continuous liquid level measurements were collected through multiple purging events to better inform the conceptual site model regarding the nature of impact at the site. Irregularities noted in these hydraulic response curves following each of the purge events are suspected to be due to the complex three-dimensional heterogeneity of the fractured/karst bedrock permeability.

Approach/Activities:

The subject 2-inch inside diameter polyvinyl chloride well was installed by a combination of sonic and wireline coring methods to the anticipated depth to water-bearing fractures. Having encountered no water, the boring was deepened approximately 20 feet below the target zone and the well was installed upon encountering a series of natural bedding plane fractures. Measurable accumulation of DNAPL in the well was observed following initial sampling events, so continuous measurement of liquid level in the well was conducted for more than three years using an In Situ Level Troll 500. During this period, DNAPL was purged from the well on multiple occasions using a weighted bailer and the transducer was left to record the hydraulic recovery curves.

Results/Lessons Learned:

Inspection of the hydraulic recovery curves suggests the presence of inflection points at a series of elevations, some of which are repeated in multiple recovery events while others are not. Data smoothing and numerical differentiation was performed to objectively identify the inflection points. Some correlation with precipitation events appears to be present, but other curve deviations appear to reflect the well construction details and potentially reflect the architecture of the DNAPL-bearing fracture system.

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Rachel Bosch