Evaluation of Indicator of Reduction in Soil (IRIS) Tubes to Verify Wetland Hydrology in Constructed Wetlands

Abstract

Wetlands are essential components of the environment that provide unique functions and values to the ecosystems in which they occur. As urban population and the economy continue to grow, impacts to wetlands are unavoidable as urban sprawl and development encroach on the surrounding natural ecosystems. Wetland restoration, enhancement, preservation, or creation projects used for compensatory mitigation for wetland impacts must go through a monitoring phase to verify the area is functioning as a wetland. Performance standards are set to verify the presence of wetland hydrology, hydric soil, and hydrophytic vegetation. Performance standards are typically monitored by methods approved for use in respective technical standards and applicable Regional Supplements to the 1987 Wetland Delineation Manual. The Hydric Soil Technical Standard details three methods for verifying hydric soil conditions, two of which are based on the reduction of iron using alpha-alpha’ dipyridyl dye or IRIS tubes. Although different methods, alpha-alpha’ dipyridyl dye and IRIS tubes serve the same purpose, which is detecting soil conditions that result in the reduction of iron; therefore, both methods should have the same influencing factors. While alpha-alpha’ dipyridyl dye is used to verify hydric soil, it can also be used to verify wetland hydrology, as it is listed as a primary indicator of wetland hydrology on the U.S. Army Corps of Engineers wetland determination data form. The goal of this project was to expand upon the original intent of IRIS tubes to determine if they can be used as a robust field method for detecting wetland hydrology, similar to alpha-alpha’ dipyridyl dye although it was not monitored as part of this study. The objectives are to find a relationship of depth and type of removal to depth of saturation or groundwater and type of hydrologic regime detected by groundwater monitoring wells. Temporal scales were investigated to determine the presence of wetland hydrology and the rate at which a positive reaction is detected on IRIS tubes. Soil parameters such as pH and organic carbon were collected to detect possible influences on IRIS tubes, and vegetation assessments were performed to assess hydrology and soil influences on vegetation establishment. Results indicate that IRIS tubes appear to be variable in assessing wetland hydrology criteria for disturbed sites throughout the duration of the growing season; however, results have a higher correlation when used during the recommended time of the growing season. This study revealed a positive relationship exists between soil water levels and removal of iron oxide paint from IRIS tubes, and that the type of iron oxide paint removal is related to the type of hydrologic regime, although temporal scales may vary with influencing factors, which should be considered when interpreting IRIS tube data.