How sewage, dams affect Lake Coeur d’Alene

Fourth in a series.

Although Lake Coeur d’Alene is the subject of a National Academy of Sciences investigation, one already-known problem doesn’t require more study: the input of nutrients into the lake water.

The nutrient problem can be summed up in a soundbite: Too many nutrients like phosphorous will change the oxygen content of the lake water, leading to algal blooms, fish kills and the release of toxic elements from the mine wastes on the lake bottom.  Major sources of nutrients are overland runoff and sewage waste streams, such as wastewater plant discharges and septic tank contamination. Overland runoff was the subject of the previous article in this series. Today’s article will deal with the problems of sewage and septic.

Septic tanks and drainfields

If every drop of sewage in the Coeur d’Alene watershed was processed by a secondary or tertiary wastewater treatment plant, the discussion on saving the lake would lack the current urgency. The tertiary treatment system for the City of Coeur d’Alene can remove 99% of the phosphorous and 90% of the ammonium nitrate from its waste stream, 99% in the summer. Unfortunately, too many lakeside homes are too distant from a treatment plant like the City of Coeur d’Alene’s.

The problem devolves to the humble septic tank and the community drainfield, both economic and effective means to address the disposal of human wastes in a wide variety of rural settings. The lakeshore was once such a place, but the population in the area has exploded and the shoreline is now paved with houses where once there were only trees.

The 2009 Lake Management Plan identified septic tanks and drainfields as a potential non-point source of nutrients. Septic tanks and drainfields are permitted through Idaho’s seven public health districts. For Lake Coeur d’Alene, the permitting agency is the Panhandle Public Health District. When the Idaho Business Review asked how many septic tanks were around the lake, the agency response was: “There is not a way for Panhandle Health District to query our database to provide a current answer.”

The standards for siting septic tanks are uniform throughout the state so there are no special rules in place for Lake Coeur d’Alene. The health district considers each permit on a case-by-case basis.

When the Idaho Business Review asked if there was a wider setback from the shoreline for septic tanks and drainfields, the public health district commented that: “The distance from drainfields (the discharging portion of a septic system) to surface water is based on the soil type, with setbacks ranging from 100 feet for low permeability soils (e.g. clay soils) to 300 feet for high permeability soils (e.g. sandy soils).  If surface water setbacks cannot be met, an easement onto a conforming property or connection to a central system is required. The setbacks to surface water are the same for all water bodies, regardless of county.  If a site does not have enough soil depth for a standard septic system, the (Idaho) Technical Guidance Manual contains a range of alternative systems that can be used to provide additional treatment of wastewater.  However, there are limitations to all alternative systems and some properties are not suitable for subsurface sewage disposal.”

Once permitted, a tank does not require any kind of annual permitting or inspection. There is also no requirement for pumping out tanks. If a tank fails, it is up to the homeowner to initiate replacement or pumping. The district does not proactively police septic tanks: “It is very rare for Panhandle Health District to need to initiate formal enforcement action. Formal enforcement actions occur approximately one-to-two times every year when other means to garner compliance are unsuccessful.”

The largest problem with septic tanks and drainfields is that no one really knows how much they contribute to the nutrient load in the lake, which is one of the motivations behind the current NAS study.

Post Falls Dam

The last of the human inputs to the lake isn’t a pollutant: it’s the human control of water levels and water flows for the lake. According to the United States Geological Survey, volume and outflow of Lake Coeur d’Alene are controlled by the Post Falls Dam on the Spokane River, which is the outlet for the lake. The dam was constructed in 1906. It raised the lake level by eight feet to provide hydroelectric power, flood control and irrigation supply. In 1940, the dam was raised another foot.

The dam gates are used to reduce outflow from the lake and to control lake level at a fixed elevation from about June to September. In September, the power company manipulates the gates to increase the outflow rates for power generation and cause the lake level to fall about 1.5 feet per month until mid-November to provide room for the spring runoff. From mid-November to May or June, the gates are fully open, and the lake seeks its natural low winter level. After spring runoff, the gates are again used to control outflow and lake level.

Raising the level of Lake Coeur d ’Alene flooded the lower reaches of the Coeur d’Alene River, which increased the rate of deposition of fine-grained mine wastes. It also made it easier for the Coeur d’Alene River to flood over its banks and deposit tailings on the surrounding lands more frequently, according the the 2005 NAS study of the Coeur d’Alene Basin Superfund Megasite report.

On the southern end of the lake, shallow areas like Hepton Lake were once wetlands or historical farmlands. Now these features are flooded in summer and fall months. The 2009 Lake Management Plan identified these now flooded shoreline areas as potential nutrient contributors to the lake.

The next article in this series will look at 40 years of federal and state regulatory actions to address the mining and post-mining impacts on the lake.