IRRIGATION WITH MUNICIPAL WASTEWATER: HOW SAFE IS IT?

Authors: Bouwer, Herman; FOX, PETER - AZ STATE UNIV, TEMPE, AZ; WESTERHOFF, PAUL - AZ STATE UNIV, TEMPE, AZ

Submitted to: Water Environment and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/3/1998
Publication Date: N/A
Citation: N/A

Interpretive Summary: More and more sewage effluent is used for agricultural and urban irrigation. So far, pathogens have been the main health concern and complete removal of viruses, bacteria, and protozoa and other parasites is required before the effluent can be used to irrigate fruits and vegetables consumed raw or brought raw into the kitchen, or parks, playgrounds and other areas with free public access. The long-term effects of such irrigation on underlying groundwater, if considered at all, usually focus on dissolved salts and nitrate. However, other chemicals must also be considered, especially since chemical concentrations in deep percolation water below the irrigated areas are much higher than the effluent itself (five times if the irrigation efficiency is 80% and rainfall is negligible). These chemicals include disinfection byproducts (DBPs), pharmaceutically active chemicals (PACHs) and DBP precursors like humic and dfulvic acids formed by decomposition of plant material (biomass). These DBP precursors form a whole new suite of DBPs when the groundwater is pumped up and chlorinated for potable use. Many of these chemicals are suspected carcinogens or otherwise toxic. While sewage irrigation thus looks good on the surface, a look at greater depths reveals potential for serious contamination of groundwater. Municipalities and other entities responsible for irrigation with sewage effluent should do a groundwater impact analysis to develop management protocols and to be prepared for liability actions. This information will benefit local and state institutions in water resources, environmental quality protection, and public health, as well as consultants and operators of effluent irrigation projects.

Technical Abstract: Using municipal wastewater for agricultural and urban irrigation is considered a safe practice if the effluent is treated to reduce virus, bacteria, and parasite concentrations to acceptable levels (zero where there is a chance for human ingestion or close contact). Effects on underlying groundwater usually are not considered. However, since the chemicals in the effluent are leached out of the root zone with much less water than was applied for irrigation, their concentrations in the deep leaching or percolation water are much higher than in the effluent, especially where rainfall is low. These chemicals not only consist of salts and nitrate, they can also include disinfection byproducts (DBPs) that were first formed by chlorination of the drinking water and then again by chlorination disinfection of the effluent, hormone and medical drug residues (pharmaceutically active chemicals or PACHs), and humic and fulvic cacids and other organic compounds formed by decomposition of plant parts (biomass) in the soil. Since the deep percolation water continues to move downward, the groundwater below effluent irrigation areas thus becomes contaminated with these chemicals. This is undesirable where the groundwater is used for drinking, since DBPs can be carcinogenic, PACHs may disrupt hormone production and weaken immune systems, and humic and fulvic acids form a whole new suite of DBPs when the groundwater is chlorinated. More research is needed to determine the severity of these issues. Meanwhile, municipalities planning to use effluent for irrigation should do a groundwater impact analysis (GIA) so as to be prepared for long-term effects on groundwater and possible liability for groundwater pollution.