Effluent Dam Compliance and Hydrogeological Assessment – Poultry Farm, Victoria, Australia

A Pollution Abatement Notice (PAN) was administered to the Poultry Farm by the Environment Protection Authority Victoria (EPA). The EPA were concerned that there was a potential for effluent to seep through the base or walls of the effluent dam to the shallow groundwater system, eventually migrating and discharging to a sensitive surface water system.

The PAN required a review of the integrity of the existing effluent dam being used to store treated wastewater from the Facility. The EPA compiled a list of actions required to address pollution concerns, which included:

  1. Constraining the distribution of aquifers and groundwater flow;
  2. Recording groundwater quality upstream and downstream of the effluent dam;
  3. Verifying that the effluent is not or has not migrated to the surface water system.

EHS Support was part of a collaborative approach with Scolexia Animal & Avian Health Consultancy that addressed these challenges by undertaking a targeted intrusive investigation, water sampling and analysis of specific indicator constituents and desktop risk assessment.

Project results and recommendations

The effluent dam at the Facility is located approximately 100 m up-gradient of a sensitive surface water system.

The shallow unconfined aquifer in the vicinity of the effluent dam is hosted by alluvial sediments, comprising low permeability silty clays, that have infilled valley sequences of the surface water systems in the area. The depth to groundwater is approximately 3 metres below ground surface between the effluent dam and the surface water system, and groundwater elevations indicated that groundwater flow was predominantly towards the surface water system.

Groundwater and surface water elevations indicated that there was a potential for groundwater to discharge to the surface water system, however this baseflow is likely to be controlled during times of higher recharge (i.e. high rainfall events when groundwater elevations are higher).

The average linear velocity was calculated to range between ~0.1 – 1 m/year, and these slow residence times are attributed to the low permeability clay lithology that comprises the shallow aquifer system.

Groundwater quality data indicated that the hydraulically downgradient monitoring well had been influenced by a source elevated in organic nitrogen, however groundwater quality data cross-gradient was not indicative of effluent water, confirming that seepage was not occurring along the length of the effluent dam.

The water quality data indicated that there was a mass reduction in Kjeldahl nitrogen between the effluent dam water quality and groundwater downgradient of the dam. The majority of this mass reduction is related to the decrease in concentration of ammonia. There is also a significant decrease in the concentration of total nitrogen. These observations are supported by the process of nitrification and denitrification within the sub-surface (un-saturated and saturated zones).

It was concluded that the source of elevated concentrations of organic nitrogen in groundwater recorded downgradient of the dam was most likely associated with the effluent dam. However, the mechanism(s) by which groundwater quality has been impacted is uncertain. One pathway could be via a mass overflow event (or events) where residual nutrient impacts in surface soils are contributing to the groundwater system via leaching. A secondary pathway could be via seepage from the base of the effluent dam. The low transmissivity of the alluvial formation indicates that it is unlikely that migration of effluent water would reach the surface water system within the timeframe of the Facilities operations however sand lenses and/or tree roots could potentially provide preferential pathways for migration. There was no evidence to suggest that the water quality within the surface water system was being influenced by the effluent dam.

Based on the outcomes of the Hydrogeological Assessment, recommendations for improvements across the groundwater well network, data analysis parameters and data collection procedures were made to ensure that potential impacts are captured and protected environmental values monitored.

The investigation methodology implemented, conclusions and recommendations were approved and endorsed by EPA.

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