Title: The impact of on-site wastewater effluent on karst springs
Researchers: Luka Vucinic, Prof. Laurence Gill, Dr Catherine Coxon
Pollution of groundwater is receiving increased attention worldwide due to widespread recognition of the facts that polluted groundwater may pose a serious threat to human health, which is a problem that is often not apparent to those affected. The purification of polluted groundwater systems may require long time and significant financial resources. The magnitude of any pollution problem depends primarily on the size of the area affected and the quantities of the pollutants involved, the various properties of each pollutant and the characteristics of rocks and soils through which any pollutant moves.
Karst terrains cover about 25% of the earth’s surface and karst aquifers are estimated to provide about a quarter of the world’s population with drinking water. However, the water quality of this important resource, in Ireland and worldwide, is subject to rapid changes as a consequence of aquifer heterogeneity which is a major concern for drinking water suppliers and users. During dry weather periods, when the diffuse flow is dominant, generally lower levels of contamination occur, but, after precipitation events (especially large ones) karst groundwater is very susceptible to contamination mainly because of point recharge to the karst conduits and little filtration within the system. The various flow systems in karst make the delineation of groundwater protection zones difficult and even if all the “hot-spots” for rapid recharge were known and included in protection zones, a certain input of diverse contaminants into the groundwater would still occur. Even brief high-amplitude increases in certain pollutants in karst catchments may have lasting detrimental effects and profound consequences at water supply springs. Effective strategies for the management and protection of water resources in karst must be based on reliable information about potential contaminant sources and transport processes, especially, in conduit systems. Due to the fact that boreholes are usually not directly connected to conduits, it is difficult to obtain trustworthy hydraulic and geometric information on conduits from borehole testing.
However, surface features of karst such as swallow holes and karst springs are likely to be linked to the conduit system, therefore, provide much more promising results. Complex pollution in karst regions is often associated with a variety of sources, thus, many studies around the world have been conducted to trace sources of pollution in karst, but direct quantification of human faecal pollution is rarely conducted.
On-site domestic wastewater treatment systems (DWTSs) are used worldwide for treatment of domestic wastewater predominantly in rural areas which are not connected to a sewage drainage scheme flowing into a large wastewater treatment plants. Globally, approximately 500 million DWTSs exist. The domestic wastewater of around one third of the population in Ireland, roughly 500.000 dwellings, is treated by on-site domestic wastewater treatment systems (DWTSs) of which more than 87% are septic tanks. Whilst the quality of spring water from karst systems has been studied in various different research projects in Ireland, few studies have actively tried to determine the relative impact of the pollutants from human on-site wastewater source. Recent studies have shown significant seasonal differences in water quality on receiving stream reaches between downstream and upstream locations adjacent to DWTSs. In addition, seasonal variations in chemical and microbial components of DWTSs effluent were observed. It is clear knowledge gaps exist on the hydrogeological transport of septic tank contaminant plumes within the receiving karst aquifer systems to discharge springs or seeps. Significant knowledge gaps exist addressing the movement of contaminants in DWTSs effluent plumes through the aerated zone of the drainage field (i.e. within the soil that is above the water table) but even more significantly through networks of fractured rock and linked conduits in aquifer systems enabling extensive transport of DWTSs effluent in karst and fractured rock aquifers well beyond 30m from source. Hence, further investigations on the mobility and natural attenuation of particles from DWTSs is very necessary in karst and fractured aquifer systems for all the reasons highlighted above.
Applying a suite of microbiological and chemical methodologies to determine contamination sources has been proven to be more effective than a single method. Therefore, this project is assessing the on-site wastewater impact on karst groundwater quality by using a range of chemical and microbiological fingerprinting methodologies (a multiple-tracer approach) and is including tracers specific to on-site wastewater sources that have been previously successfully used in Ireland and worldwide (although not on karst springs), as well as some novel techniques that are being trialed, especially, in terms of applicability, sensitivity and accuracy (e.g. flow cytometry, fluorescent whitening agents).
The overarching aim of this research is to carry out a focused systematic investigation on the impact of particulates from DWTSs in karst systems on the water quality of associated discharge springs to better understand the risk to both groundwater pollution and human health in an Irish context but with wider international applications. The research activities should determine the impact of DWTS effluent discharging to springs in rural karst catchments for the first time in Ireland. This will enable appropriate strategies to be implemented to protect both the environment (for example, a programme of measures as required by the Water Framework Directive) and/or the public’s health. Additionally, findings from this project will be beneficial on the international scale, by evaluating several techniques in different karst environments and under specific (climatic and hydrogeological) conditions, and might offer valuable experience with applying a suite of various methodologies to determine contamination sources.
This research will answer questions related to the applicability of some of the selected suite of different techniques. Since it is known that many methods now exist for detection of low concentrations of contaminants of anthropogenic origin, but a large number of them are, either slow and complex, or require expensive equipment and highly trained personnel, this project is assessing the possibility for overcoming such issues in karst aquifer systems by pioneering the use of non-conventional tracers to establish links between subsurface particulate movement through karst conduits from DWTSs effluent to emerging discrete outlet springs. Additionally it will enable characterization of DWTS flow patterns within receiving karst groundwater systems over a range of hydrological events and examination of the factors causing the high levels of particulates discharged in the effluents from DWTSs across a range of karst systems under variable hydrological conditions in Ireland. Some particulates are being characterized in terms of geochronology in order to establish the particulates residence time within counduit/fracture networks. Also, this will enable identification and differentiation of DWTSs sources within catchments and biogeochemical processes impacting attenuation along the conduit/fracture networks