UNDERSTANDING THE EFFECT OF ROADWAYS ON THE MOVEMENT OF GROUNDWATER THROUGH PEATLANDS AND POTENTIAL IMPACTS TO SALMON HABITAT

Salmon are critical to life on the Kenai Peninsula, AK. As the population continues to grow so does the new infrastructure needed to make more remote areas of the Peninsula accessible. Numerous roads now bisect wetlands and salmon streams. Groundwater contribution to salmon rearing streams has been identified by local agencies as an important but poorly studied habitat feature. This study integrates physical and chemical hydrogeologic methods to assess the impact of a road on shallow groundwater movement in a wetland and on baseflow to a stream. Two sites, one with a road perpendicular to the stream and one with a ditched road parallel to the stream, were instrumented with water table wells, piezometers, and a stilling well. From this monitoring network regular water level measurements, field chemistry sampling, and δ¹⁸O and δ³H stable isotope analyses were utilized to assess hydrogeologic processes and flowpaths. Significant pooling of the water table was observed on the upgradient side of the road relative to the downgradient side. Conversely, road ditching lowered the water table and increased the temperature of the intercepted groundwater prior to stream entry. Reduced hydraulic head in streambed minipiezometers was observed at the road crossing. A reduction in upgradient storage capacity due to the elevated water table results in enhanced flooding at the crossing even when the channel upstream remains below bankfull. In addition to a conceptual model these data will be used to constrain a numerical model of groundwater flow though the study section. Together, the monitoring and modeling can be used to both improve understanding of current conditions and to simulate effects of mitigation measures, identify recommendations for future road assessments, and suggest methods of impact reduction. This information will provide the basis for more general management tools for local land managers, stakeholders, and policy makers.