Long-term changes in riparian connectivity and groundwater chemistry in an urban watershed

Hydrologic alterations associated with urbanization can weaken connections between riparian zones, streams, and uplands, leading to negative effects on the ability of riparian zones to intercept pollutants carried by surface water runoff and groundwater flow such as nitrate (NO₃⁻) and phosphate (PO₄³⁻). We analyzed the monthly water table as an indicator of riparian connectivity, along with groundwater NO₃⁻ and PO₄³⁻ concentrations, at four riparian sites within and near the Gwynns Falls Watershed in Baltimore, MD, from 1998 to 2018. The sites included one forested reference site (Oregon Ridge), two suburban riparian sites (Glyndon and Gwynnbrook), and one urban riparian site (Cahill) with at least two locations and four monitoring wells, located 5 m from the center of the stream, at each site. Results show an increase in connectivity as indicated by shallower water tables at two of the four sites studied: Glyndon and Cahill. This change in connectivity was associated with decreases in NO₃⁻ at Glyndon and increases in PO₄³⁻ at Glyndon, Gwynnbrook, and Cahill. These changes are consistent with previous studies showing that shallower water table depths increase anaerobic conditions, which increase NO₃⁻ consumption by denitrification and decrease PO₄³⁻ retention. The absence of change in the forested reference site, where climate would be expected to be the key driver, suggests that other drivers, including best management practices and stream restoration projects, could be affecting riparian water tables at the two suburban sites and the one urban site. Further research into the mechanisms behind these changes and site-specific dynamics is needed.