Land Application of Treated Wastewater within the Black Belt of Alabama
Type of DegreeMaster's Thesis
Civil and Environmental Engineering
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The Black Belt of Alabama is a region of the United States that has seen its population decrease and poverty rise over the last century. The Black Belt was a thriving part of Alabama due to the rich, fertile soil. The dark soil’s color was the cause of the area being called the “Black Belt”. The dark soil known as the Blackland Prairie soil is a shrink-swell clay in the vertisol family. The Blackland Prairie soil has a low percolation and conductivity rate. The soil has made it difficult for land application sites in the Black Belt to be effective in draining discharged treated wastewater. The Black Belt currently has six land application sites, or commonly referred to as “spray fields”, located throughout the region. Two of the six spray fields had ten or more NPDES violations, between 2018 and 2020. Two of the remaining four have had a history of NPDES violations, at least three violations between the years 2018 and 2020, and the final two spray fields have had no NPDES violations. Currently there are eighteen spray fields throughout the entire state. One of the spray fields within the Black Belt is in Uniontown, AL. The spray field in Uniontown has had compliance issues with the Alabama Department of Environmental Management (ADEM) dating back to 2006. The spray field is currently severely flooded due to the spray field being undersized relative to the permitted discharge. The spray field currently has a discharge permit of 1,893 m3 d-1 (500,000 gallons d-1). The spray field regularly exceeds the permitted discharge. The recorded monthly average discharge has been reported as high as 6,284 m3 d-1 (1,660,000 gallons d-1). Groundwater modeling by MODFlow Flex found the spray field site would only be able to work properly if discharged wastewater effluent traveled via the subsurface to Freetown Creek located 330 meters to the west of the discharge sprinklers. The spray field site was found to be able to work properly if the permitted discharge was reduced by 55% to 852 m3 d-1 (225,000 gallons d-1) as opposed to 1,893 m3 d-1 (500,000 gallons d-1), the current permitted discharge. The 1,893 m3 d-1 discharge permit would only be viable for the existing spray field if the hydraulic conductivity of the upper soil layer was almost two orders of magnitude higher. The existing spray field area was found to be undersized based on the loading rates relative to the spray field’s capacity.