Geophysical and hydrogeological characterisation of the impacts of on-site wastewater treatment discharge to groundwater in a poorly productive bedrock aquifer

Abstract

Contaminants discharging from on-site wastewater treatment systems (OSWTSs) can impact groundwater quality, threatening human health and surface water ecosystems. Risk of negative impacts becomes elevated in areas of extreme vulnerability with high water tables, where thin unsaturated intervals limit vadose zone attenuation. A combined geophysical/hydrogeological investigation into the effects of an OSWTS, located over a poorly productive aquifer (PPA) with thin subsoil cover, aimed to characterise effluent impacts on groundwater. Groundwater, sampled from piezometers down-gradient of the OSWTS percolation area displayed spatially erratic, yet temporally consistent, contaminant distributions. Electrical resistivity tomography identified an area of gross groundwater contamination close to the percolation area and, when combined with seismic refraction and water quality data, indicated that infiltrating effluent reaching the water table discharged to a deeper more permeable zone of weathered shale resting on more competent bedrock. Subsurface structure, defined by geophysics, indicated that elevated chemical and microbiological contaminant levels encountered in groundwater samples collected from piezometers, down-gradient of sampling points with lower contaminant levels, corresponded to those locations where piezometers were screened close to the weathered shale/competent rock interface; those immediately up-gradient were too shallow to intercept this interval, and thus the more impacted zone of the contaminant plume. Intermittent occurrence of faecal indicator bacteria more than 100m down gradient of the percolation area suggested relatively short travel times. Study findings highlight the utility of geophysics as part of multidisciplinary investigations for OSWTS contaminant plume characterisation, while also demonstrating the capacity of effluent discharging to PPAs to impact groundwater quality at distance. Comparable geophysical responses observed in similar settings across Ireland suggest the phenomena observed in this study are more widespread than previously suspected.

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“…However, Beal et al (2005) stress that the degree of attenuation experienced strongly depends on the thickness of the unsaturated zone separating the infiltrating area, while the hydraulic gradient and subsoil saturated/unsaturated permeability determine capacity of the ground to accept infiltrating effluent. Investigations by Donohue et al(2015) in comparable deposits to those encountered in the Mattock Catchment have noted that elevated water tables of perched groundwater can lead to effluent exfiltration in a setting where the OSWWTS is underlain by clayey till and a poorly productive bedrock aquifer. McCarthy et al (2012) noted that in these settings it is common practice for property owners to install overflow pipes from OSWWTS percolation areas to open drains to prevent exfiltration.…”

“…The rise of approximately 20 cm is small, but may be sufficient to bring (regional and perched) water tables up to the base of OSWWTS percolation areas (as noted by Donohue et al, 2015)). Moreover Moe et al (2010) report that groundwater fluctuations in Irish poorly productive aquifer systems generally increase from discharge points toward catchment divides.…”

“…Although these systems have been demonstrated as effective technologies for wastewater treatment in free draining Irish subsoils ( (Keegan et al, 2014), their efficiency in lower permeability deposits where high water tables limit treatment capacity, including the removal of FIOs, have proved more questionable (Donohue et al, 2015). Characterizing attenuation processes associated with OSWWTSs has proved particularly challenging in geologically heterogeneous catchments, where highly variable subsoil cover complicates characterization of the capacity of the ground to accept percolating effluent for part or all of the year.…”

“…Characterizing attenuation processes associated with OSWWTSs has proved particularly challenging in geologically heterogeneous catchments, where highly variable subsoil cover complicates characterization of the capacity of the ground to accept percolating effluent for part or all of the year. Where ranges of groundwater fluctuation prove substantial, this may result in intermittent direct discharge to the surface, potentially resulting in significantly reduced levels of effluent treatment before reaching aquatic receptors (Donohue et al, 2015).…”

Imageamento Elétrico Como Ferramenta De Monitoramento Do Comportamento Da Água No Solo

“…Desse modo, o método elétrico de corrente contínua tem se mostrado adequado na identificação e delimitação de plumas de contaminantes (Nascimento et al, 2004;Eunice et al, 2011;Donohue et al, 2015) e estudo do comportamento da água nas zonas não saturada e saturada do solo (Asry, 2012;Calamita et al, 2012;Chukwauma et al, 2015). Além disso, a correlação entre a resistividade elétrica e outras propriedades físicas do solo, tais como condutividade hidráulica e umidade, podem ser observadas nos trabalho de Knight et al (2010) e Brillante et al (2014).…”