Here’s some background info on the Fourmile fire.
One year after the devastating Fourmile Canyon fire, scientists are still trying to determine the full impact of the burn on the local water supply.
Some findings are already in: As summer thunderstorms pounded the scorched land in July, mud and ash were washed into Fourmile Creek, potentially contaminating the water with chemicals and heavy metals and affecting the drinking water of those downstream.
“The stream was black” noted United States Geological Survey (USGS) hydrologist Sheila Murphy, recalling the condition of Fourmile creek during an intense thunderstorm in early July.
“Our goal is to capture data during these storm events,” Murphy said. So she and her USGS co-workers have installed a handful of automated monitoring stations along Fourmile Creek that take samples during and after significant rainfall.
So far, her research has shown that erosion is particularly severe during big thunderstorms.
Samples are analyzed for a number of indicators of water quality. One is “turbidity,” a measure of how much light can pass through the sample. The more sediment in the water, the more light is scattered by the particles,. Water providers have quality standards that are directly related to turbidity. Large amounts of sediment in the water prevents effective disinfection, and the suspended particles provide attachment points for chemical contaminants.
The samples are also analyzed for a suite of chemicals, including partially burned carbon compounds, soil nutrients like nitrates and phosphates, and dangerous heavy metals like lead and arsenic, which can be washed away from old mine tailings in the region freshly exposed by the fire.
The findings are not yet complete, but from samples already collected Murphy has documented significant effects. Water samples taken upstream of the burned area during storms show low turbidity: a bit cloudy but still translucent. In stark contrast, samples from within and downstream of land scorched by the fire showed extremely high turbidity during two big thunderstorms in July. The water samples were nearly opaque with the soil and silt that has washed into the stream.
The impact of the storms on water quality was sudden and severe, but the effects persisted. “Some of the July storms moved so much sediment to the stream that we saw elevated turbidity levels for weeks.”
In addition, the storms caused the concentration of dissolved materials to spike in the water. But she has not yet completed an analysis of the exact chemical composition of the materials.
Thunderstorms are damaging because the intensity of the raindrops hitting the exposed soil kicks up and frees large amounts of sediment. Just how much can be seen near one of her monitoring sites, where a partly burned area directly adjacent to the creek measuring about 20 by 50 feet is blanketed in a thick layer of dried mud and sand that washed down from surrounding slopes during a storm earlier this summer.
The damaging effect of fires on watersheds is not unknown to water providers in the area. Years after the 2002 Hayman Fire burned 138,000 acres southwest of Denver, Denver Water is still dealing with mud and debris that wash into a number of reservoirs, Murphy said. The work is on-going and costly.
As reported by the Denver Post, the Hayman restoration project is attempting to raise $4.6 million to revegetate burned slopes that are still destabilized by the nine-year old fire. While the Fourmile fire was smaller in size, and Fourmile creek does not feed directly into any reservoirs, the creek is a source of water for Pine Brook hills, a community of 400 homes just west of Boulder.
Murphy hopes that the information gathered here will assist communities in future burn zones. “If you are a [water treatment] plant removing a lot of water from the stream and you get these huge sediment loads to your plant or reservoir – that really impacts your ability to treat your water to a level that is good for households.”