In a lab at the Fleur Drive Treatment Plant, Des Moines Water Works chemist Lisa Morarend filtered samples of river water into vials. She then put them in an instrument that measures nitrate at 1/100 of a milligram per liter (mg/L), along with other contaminants, in 18 minutes.
Morarend pointed to the latest results on a computer screen. A small peak indicated salt. The larger one showed nitrate.
The U.S. Environmental Protection Agency (EPA) set the legal limit of nitrate in public drinking water at 10 mg/L to protect against “blue baby syndrome,” a potentially fatal condition for infants.
On Tuesday, Central Iowa Water Works measured nitrate in the Raccoon River at 14.17 mg/L. The Des Moines River clocked in 13.9 mg/L.
“We have to remove that somehow to get below that 10 [mg/L] so that we can ensure that we have safe drinking water for the metro and surrounding communities,” Morarend said.
Inside Des Moines' nitrate removal facility
The Fleur Drive Treatment Plant added a nitrate removal facility in the early 1990s. At the time, it was the largest in the world. It runs only when needed. Months, sometimes years can pass without it being operated.
Central Iowa Water Works Executive Director Tami Madsen said Tuesday that the nitrate removal facility had run “more than 65 days” so far this year. It costs around $16,000 per day to operate.
While highly effective, the nitrate removal facility has capacity for 10 million gallons a day. Morarend explained that’s a fraction of normal demand, which can be 70-80 million gallons.
Water that passes through the nitrate removal facility is blended with water that goes through the normal treatment process to meet the federal drinking water standard. Morarend said this is easier when nitrate in the rivers is 11 mg/L, rather than 20.
On June 8, the Raccoon River hit 20.55 mg/L. A Central Iowa Water Works spokesperson said it marked the second time nitrate in the river surpassed 20 mg/L. The record was set in 2013 with 24.39 mg/L.
On June 12, nitrate in the Des Moines River reached 17.15 mg/L — below the record 18.62 mg/L in 2013.
“That's a lot of nitrate that we have to remove, and that strains our capacity,” Morarend said.
Central Iowa Water Works issued a lawn watering ban June 12 to ease that strain and keep pace with nitrate removal at this plant, along with other plants that use reverse osmosis.
“Thankfully, the lawn watering ban has been incredibly effective, and so we have not exceeded the drinking water standard of 10 mg/L. So, we do not anticipate needing to put any kind of further restriction or ban whatsoever,” Madsen said.
Morarend said remote sensors in different stages of the treatment process track nitrate levels, but running samples through ion chromatography remains the gold standard. Throughout the day, staff use quality control samples with known levels of each of the contaminants to ensure accuracy.
Treated water leaving the Fleur Drive plant Tuesday was 8.01 mg/L.
Challenges of using surface water
Many Iowa communities use groundwater, which tends to have fewer nitrates and is easier to treat compared to river water, according to Morarend.
“Unfortunately, for the bigger utilities like us, we can't get enough water out of the ground. We would use up all of the groundwater that's down there, so we have to use surface water, in particular, these two rivers,” she said. “That leads to some unique challenges for us because, as you've seen with the nitrate levels going up and down, the water quality in the rivers can change within hours.”
The Des Moines and Raccoon rivers drain two watersheds in north-central Iowa, which has a high concentration of row crops and field tile drainage.
“Everything that's coming off the landscape in that area is washing down to us, and it's something that we need to be concerned about,” Morarend said.
Nitrate is both a naturally-occurring and synthetically-made chemical compound that’s essential to plant and animal growth. During droughts, it can accumulate in the soil and flush out in high concentrations with rain.
Understanding what’s happening upstream can help treatment plants prepare and adjust as needed, according to Morarend.
“We have partnered with the United States Geological Survey to put nitrate analyzers in our source water rivers,” Morarend said. “If it rains up in Sac City, we can see how that affects the nitrate levels in the river there and then we can watch that nitrate as it moves down the river to us.”
Nitrate levels in the Des Moines and Raccoon rivers were also high in the spring of 2024, when rainfall ended a four-year drought in Iowa. But Morarend said the utility experienced higher demand this spring before the lawn watering ban and the nitrate levels have been elevated for a longer period of time in both rivers.
She's hopeful levels will naturally decrease as the summer progresses, similar to years past.
