AN EVALUATION OF THE PRACTICALITY OF USING THE "RUSTY ROD" TECHNIQUE TO ESTIMATE GROUNDWATER LEVELS IN CONSTRUCTED WETLANDS IN NORTHERN VIRGINIA

Abstract

The "rusty rod technique" was field-tested to address its suitability as a substitute for shallow groundwater monitoring wells to evaluate groundwater depth in wetlands. The field test consisted of randomly emplacing iron rods into the ground around shallow groundwater monitoring wells in wetlands and uplands during the growing season. A portion of the rods were removed part way into the field test (at least four weeks); the remainder were left in place for up to 10 weeks. The depth of below-ground rod rust was compared with groundwater depth measurements at each well. Two natural and two constructed wetlands were used. The field tests were conducted late in the 1990 growing season and early in the 1991 growing season. The rods developed below-ground rust in wetlands but not in uplands. A band of contiguous rough rust extended for some distance below the ground surface on all of the wetland rods. An additional band of uneven patchy rust extended for some distance below the contiguous rust band on the rods in the natural wetlands. Rod rust depth did not uniformly equal average groundwater depth, or the deepest or shallowest groundwater depths. In the natural wetlands, which had minimal groundwater fluctuation during the field tests, the lowest limit of the patchy rust was near the deepest groundwater depth. In the constructed wetlands, which had groundwater fluctuation throughout the field tests, the lowest limit of the rod rust was within the shallow part of, or above, the range of groundwater measurements. The rod rust depth appeared to reflect some groundwater depth trends. Locations with deeper groundwater depth measurements tended to have deeper rod rust than the other locations, and locations where the depth to groundwater as increased during the field test, tended to have deeper rust on the long-term rods than on the short-term rods; however, there were enough inconsistencies to suggest that conditions other than groundwater depth were influencing rod rusting. Additionally, rust depth was not consistent with groundwater depth measurements in indicating wetland hydrology. The rusty rod technique would not be an accurate substitute for groundwater monitoring wells to determine actual or average groundwater depth, or to determine the presence of wetland hydrology.