Three years ago I wrote an article describing the importance of phosphorus (or rather its absence) in our Priest River/Lake watershed. Our lake and streams are generally crystal clear because the waters contain very minimal concentrations of nutrients. This makes it difficult for algae and aquatic vegetation to thrive. Since that time we have taken dozens of water samples from the lake as well as every major tributary stream. In addition this spring a Ph.D. thesis from University of Idaho examined the role of sewer districts in the management of nutrients such as phosphorus and nitrogen. So now is a very good time to review what we know about these nutrients and the issues they may create in our environment.
To briefly review, both nitrogen (N) and phosphorus (P) are quite scarce in our Priest environment. Weathering rock is ordinarily the natural source of P, but our igneous bedrock contains virtually no P. In the east side streams, especially the northern streams, we generally find that P levels are undetectable. Another source of P is rotting vegetation. This is probably why we find mildly elevated levels in some of the west side streams such as Kalispell and Reeder which both derive considerable water from wetlands such as Bismarck Meadows. Eroded soil will carry P that is adsorbed to soil particles but this is generally unavailable to plants. Finally fertilizer probably was a significant contributor to the lake P in the past. However, since 2017 Washington has banned P in fertilizers labeled as lawn fertilizer. Since North Idaho receives shipments from Washington warehouses, we no longer likely have much P derived from lawn fertilization.
Nitrogen is principally derived from atmospheric molecular N2. This N is unavailable to plants until nitrogen fixing bacteria chemically convert it into nitrates and nitrites.
Finally, large quantities of N and P are imported every month of every year by visitors to the lake and lake residents. These nutrients come from our groceries. Food is processed in our bodies (we need these nutrients too) and eliminated into our toilets. From there the wastewater is processed initially in a septic tank. Then it is pumped up to sewage lagoons where bacteria consume both P and N. The bacteria cannot remove all the nutrients so the remaining wastewater is irrigated out onto a plot of native vegetation, mostly consisting of conifers. The assumption has been that the trees will take up some of the nutrients and the soil will bind the rest.
This process was examined in research done by Eureka Joshi at the University of Idaho. Her thesis was published recently. In this study five districts, similar in every way to ours, located near Lake Pend Oreille and Lake Coeur d’Alene were evaluated. Some districts were relatively new although others had been in operation for 20-45 years. In brief she found that in the early years the native conifers did a very good job of absorbing nutrients and this resulted in measurable growth. However, over the course of decades the effect waned. In time the soils began to show evidence of saturation, particularly to N. She estimates that this could begin to be an issue around 22 years of service. So far most of the P seems to be bound to the soil, but the soil’s capacity is not infinite. It appears that after a period of time, as yet to be determined, the irrigation plots will have to be retired and new land found. Mark Coleman, Eureka’s supervisor at U of I, wants to confirm these findings and hopes to include some of our Priest Lake districts in these studies. This would provide very useful information to help guide further operation and planning of our current sewer districts.
We continue to observe plumes of aquatic vegetation growing over the course of the summer at groundwater outflows all along Kalispell Bay. For a number of years we have observed elevated levels of P in shallow groundwater wells near the lake. Since the P from our wastewater appears to be effectively bound by soils at the district sites, I asked Dr. Coleman what he thought might be the cause of our problems at lakeside. Without hesitation he indicated it is most likely due to leaky sewage pipes. In this regard it is important to note that at Kalispell Bay and Coolin the oldest PVC sewage lines are 50 years old. This is their rated lifetime. Clearly the time has come to begin replacing our oldest lines, starting with the ones in closest proximity to the lake.