Hydrologic influence of wetland restoration : the Story Mill case study

Abstract

The Story Mill Wetland is a 20 hectare restoration project in Bozeman Montana, intended to help improve the quality of surface water that leaves the city. The streams that border the property, Bozeman Creek and the East Gallatin River, exceed the Montana Department of Environmental Quality's (MTDEQ) water quality standards for nitrogen (0.27 mg/L) and phosphorus (0.08 mg/L). Wetlands in the landscape have become more intriguing in the advent of MTDEQs adoption of Circular DEQ-13, a legal framework for nutrient trading to achieve improved watershed water quality. Earth-work took place in Summer/Fall 2014, including excavating 5,800 m 2 of disconnected floodplain, and filling a surface drain. The research objectives were to quantify the impacts of the restoration so as to make inferences about the short-term changes in groundwater/surface water interaction, wetland volume and area, and the wetland's impact on the water quality in the bordering streams. Measurements of groundwater levels and surface water flow rates, and water chemistry analyses for both water sources, were recorded weekly from 30 shallow wells and 5 stream gauging stations from August 2014 through September 2015. Groundwater velocity and hydraulic residence time were estimated by performing slug tests in several groundwater wells. Spatially normalized wetland area and volume were calculated based on interpolated groundwater surfaces. Throughout the monitoring period, in all surface and groundwater samples, total nitrogen never exceeded 3 mg/L, averaged 0.76 mg/L, and almost always exceeded the target standard for the East Gallatin River. Total phosphorus was below the detection limit in 97% of all samples and never exceeded 0.22 mg/L. Neither average nutrient concentrations nor pH showed significant general temporal trends, while dissolved oxygen decreased over time. Changes in hydrology were generally localized near earth moving activity. Overall, wetland volume decreased slightly and wetland area increased slightly. Hydraulic gradients showed the primary flow of groundwater to be out of the wetland, with an average soil water velocity of 0.11 m/day. The slow moving groundwater in the wetland system appears to limit the extent of groundwater/surface water interaction, and hinders the role of the wetland in enhancing the water quality in the receiving creeks.