Portneuf River (Idaho)

Nutrient runoffs from intensive agriculture and ranching along the route of the river have noticeably increased the nitrate loads in the stream. This is compounded by the addition of treated effluent from the city of Pocatello's wastewater treatment facility and the waste associated with the phosphate processing plant, Simplot. This increased nutrient load supports an increased biomass within the system.[10] The DEQ in collaboration with the City of Pocatello, the Soil Conservation Commission, and USDA are developing tools and practices designed specifically to alleviate the nitrogen issues associated with agriculture and its waste. Known as Best Management Practices (BMPs), these methodologies are designed and modified for varying situations and have proven themselves effective through prior implementation projects.[11]

The exchange of inorganic carbon between the solid earth and the atmosphere in the Portneuf watershed has produced deposits of CaCO₃ throughout the system. Known as travertine and tufa, these deposits are formed as a function of the dynamic groundwater and geology of the region. Tufa is known throughout the geologic collective as the soft and porous CaCO₃ deposits associated with moving freshwater environments.[12] Travertine is a related deposit separated by the fact that it is associated with thermal waters.[13] Deposition of tufa is complex, involving processes of dissolution, saturation, subsurface transport, emergence and precipitation.[14] Both versions of precipitated CaCO₃ are present in the Portneuf watershed. Several very different but related processes control the precipitation of CaCO₃ in natural aqueous systems. The chemical processes are driven by the chemical properties of the elements and molecules involved. The physical processes are driven by characteristics of the watershed system (gradient, flow, substrate, groundwater flow properties). The biotic processes are driven by the activities of living organisms.

Calcium carbonate formations are associated with regions where meteoric waters become enriched with calcium carbonate by direct dissolution of CaCO₃-rich rocks underground only to resurface and re-precipitate calcite. It is well known that the geology of the Portneuf watershed contains large thicknesses of limestone and dolomite strata that are mostly Paleozoic.[10][15] Enrichment of CaCO₃ in the meteoric waters is due to the addition of CO₂ either through interaction with the atmosphere, or infiltrating through organic compound containing soil layers. This saturation of CO₂ in the groundwater allows for the dissolution of carbonate rocks as it drives pH down.[12] As the water reemerges it is exposed to the atmosphere and the concentration gradient of CO₂ associated with it. As the enriched waters attempt to achieve equilibrium they precipitate calcite,[14] through the reaction of Ca⁺² + 2HCO⁻ ⇔ CO₂↑ + H₂O+ CaCO₃↓. The mountains surrounding the Portneuf are characterized by thick Paleozoic limestone deposits rich in carbonates,[15] and many springs do emerge along the reach associated with Lava Hot Springs.

Another factor affecting the precipitation of tufa is the physical aspects of the river system. The gradient of the Portneuf through Lava Hot Springs is such that this reach is dominated by a series of riffles with some larger falls. This turbulence and increased surface area caused by aeration facilitate the out gassing of CO₂, thus increasing the saturation of CaCO₃ to the point of precipitation. This is a phenomenon noticed throughout the world as the development of waterfall tufa[16] and is a relevant explanation for at least some formation through the region, especially through the town site of Lava Hot Springs. The figure below is a schematic representation of the stream gradient of the Portneuf from just below Chesterfield Reservoir and above American Falls Reservoir adapted from Minshall, 1973.

The third recognized mechanism for tufa development is the active role played by the biota. Algae and mosses, along with higher plants and some insects, often trap tiny particles within their sinuous roots, fronds, and shelter/feeding structures, acting as nucleation points for further precipitation.[16] This may explain some of the deposition locations, but the biota plays a larger role as photosynthetic plants remove CO₂ from the water, further concentrating the Ca²⁺ and CO₃²⁻ and driving precipitation. The Portneuf through this reach is rich in plant life not only due to the nutrient accumulation as it travels through farmland but from the warm water inputs that protect the stream from the effects of the harsh cold of winter temperatures. This could be a mechanism for potentially increasing tufa formation through the reach. An interesting study completed in 1972 showed evidence that the development of tufa and travertine filled the interstices normally found on the rocky river. This had implications for some burrowing organisms as well as nutrient cycling.[10]

Precipitation of tufa in the Portneuf drainage is produced through the combination of four complex mechanisms: dissolution of limestones by meteoric waters containing carbonic acids, degassing of CO₂ at turbulent sites, the removal of CO₂ by photosynthetic plants, and the trapping of particles of CaCO₃ by the biota. The complex interactions between these different mechanisms may never be fully understood but offer insight into the occurrence of formations.