Risk Assessment for Farmland to Wetlands Conversions

Ecology and ecohydrology issues play key roles in assessing legacy pesticides and residual nutrient effects on mitigation projects converting former farmland to wetlands.

The San Diego County Water Authority (SDCWA) has received environmental permits to reclaim 40-acres of former farmland in Tijuana River Valley to establish native wetlands habitat as mitigation for capital improvement projects to the regional conveyance system.

“Risk assessment is significant for any agricultural-to-wetlands conversion where residual pesticides and/or nutrients have the potential to re-enter the ecosystem and potentially harm existing fragile wetlands,” said Peter Quinlan, Dudek’s hydrogeology practice manager who led the team on the risk assessment project.

Key to regulatory approval was preparing a defensible ecological risk assessment for legacy pesticides and evaluation of residual nutrient effects for the Regional Water Quality Control Board (RWCQB).

“Chlorinated pesticides were commonly used on farmland in the mid-20th Century and can be toxic to animals, especially aquatic life such as benthic macro-invertebrates at much lower concentrations than humans,” Quinlan said.

Quinlan said high concentrations of nitrogen and phosphorus entering a water body, such as the project site’s coastal estuary, can cause excessive algae growth, deplete available oxygen in the water, and eventually be toxic to fish.”

As a 303(d)-listed impaired water body, the valley’s coastal estuary receives additional protection under regulations administered by the RWQCB. In addition, the federally-listed endangered species least Bell’s vireo is present on the site and any potential negative effects had to be identified prior to permit issuance and project implementation

Dudek fielded a multi-discipline team of engineers, hydrogeologists, biologists and wetlands experts to look at the potential for residual pesticides to threaten mammalian, aquatic, or avian life.

Dudek scientists conducted a toxicology risk assessment to evaluate several exposure scenarios; review impacts to burrowing mammals and birds within the agricultural fields of proposed wetland area and adjacent berms; and review impacts to plants, fish, and birds in the downstream coastal estuary.

Evaluating impacts to the downstream estuary considered sediment transport of adsorbed contaminants and overland and subsurface flow of dissolved contaminants. The evaluation considered how the adsorbed and dissolved concentrations would be diluted by joining river flows.

“The studies found that the pesticide concentrations still present would not be mobilized in sufficient quantity to raise concentrations in the estuary to a dangerous level,” Quinlan said.

The study also found that the soluble concentrations of nutrients detected in the agricultural field would not result in elevated nutrient concentrations in the estuary. Additionally, the proposed wetland would serve to reduce nutrient concentrations through vegetation uptake and denitrification.

The RWCQB approved use of the agricultural field for the wetlands project based on the findings of ecological risk assessment and nutrient concentration calculations, and an agreement to remove three areas with elevated levels of pesticide contamination.

Another consideration now for this type of project is the new Benthic Macroinvertebrate (BMI) monitoring requirements that put new emphasis on the health of aquatic ecological systems, he said.

Quinlan said preparing a defensible ecological risk assessment for legacy pesticides and residual nutrient effects requires understanding three factors based on ecology and ecohydrology:

  • Use appropriate species that effectively act as surrogates for local onsite species.
  • Understanding ecological pathways that can facilitate pesticide movement into the ecosystem.
  • Knowledge of surface (hydrology) and subsurface (groundwater) mechanisms that affect ecological pathways.