Groundwater in Fractured Bedrock Environments: Managing Catchment and Subsurface Resources
Fractured bedrock aquifers have traditionally been regarded as low-productivity aquifers, with only limited relevance to regional groundwater resources. It is now being increasingly recognised that these complex bedrock aquifers can play an important role in catchment management and subsurface energy systems. At shallow to intermediate depth, fractured bedrock aquifers help to sustain surface water baseflows and groundwater dependent ecosystems, provide local groundwater supplies and impact on contaminant transfers on a catchment scale. At greater depths, understanding the properties and groundwater flow regimes of these complex aquifers can be crucial for the successful installation of subsurface energy and storage systems, such as deep geothermal or Aquifer Thermal Energy Storage systems and natural gas or CO2 storage facilities as well as the exploration of natural resources such as conventional/unconventional oil and gas. In many scenarios, a robust understanding of fractured bedrock aquifers is required to assess the nature and extent of connectivity between such engineered subsurface systems at depth and overlying receptors in the shallow subsurface.
Metolachlor dense non-aqueous phase liquid source conditions and plume attenuation in a dolostone water supply aquifer
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Published:January 01, 2019
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CiteCitation
Beth L. Parker, Kenley Bairos, Carlos H. Maldaner, Steven W. Chapman, Christopher M. Turner, Leanne S. Burns, James Plett, Raymond Carter, John A. Cherry, 2019. "Metolachlor dense non-aqueous phase liquid source conditions and plume attenuation in a dolostone water supply aquifer", Groundwater in Fractured Bedrock Environments: Managing Catchment and Subsurface Resources, U. Ofterdinger, A.M. MacDonald, J.-C. Comte, M.E. Young
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Abstract
This paper reports a long-term field investigation of a fractured dolostone aquifer that was penetrated by a dense non-aqueous phase liquid. High-resolution source zone characterization shows the evolution of deep penetration to the back-diffusion conditions seen at the present day. Metolachlor, a common herbicide, was released into the overburden overlying a fractured dolostone aquifer within a short time window (1978–81). In 2000, the plume front arrived at a municipal supply well located 930 m down-gradient, increasing to a maximum concentration of 2 μg l−1. Groundwater monitoring with high-resolution, depth-discrete multi-level sampling systems since 1992 shows a clearly delineated bedrock plume. Numerous rock core samples show metolachlor in the low-permeability rock matrix at the bottom of the aquifer. The mass distribution and bedrock hydraulic head pattern strongly suggest that metolachlor entered the bedrock as a free-phase dense non-aqueous phase liquid penetrating to the aquifer bottom, preferentially accumulating in some horizontal fractures, dissolving quickly as a result of the rapid groundwater flow and then diffusing into the rock matrix, where back-diffusion sustains a dilute, persistent and stable plume. Strong plume retardation by matrix diffusion and sorption has greatly mitigated the impact on water quality in the down-gradient supply well, allowing for its continued use, while back-diffusion and degradation maintain a persistent, dilute plume managed by appropriate monitoring.
- aquifers
- attenuation
- bedrock
- boreholes
- carbonate rocks
- chemical properties
- concentration
- contaminant plumes
- cores
- dense nonaqueous phase liquids
- diffusion
- dolostone
- fractured materials
- fractures
- ground water
- herbicides
- hydraulic conductivity
- hydraulic head
- levels
- metolachlor
- monitoring
- nonaqueous phase liquids
- numerical models
- organic compounds
- permeability
- pesticides
- physical properties
- pollutants
- pollution
- sedimentary rocks
- sorption
- theoretical models
- transport
- water management
- water quality
- water supply
- water wells
- well-logging