Citation:

Wood, K., S. Kaushal, P. Vidon, P. Mayer, AND J. Galella. Tree Trade-Offs in Stream Restoration: Impacts on Riparian Groundwater Quality. Urban Ecosystems. Springer Science+Business Media B.V, Dordrecht, Netherlands, 25:773-795, (2022). https://doi.org/10.1007/s11252-021-01182-8

Impact/Purpose:

Riparian zones are a vital interface between land and stream and are often the focus of stream restoration efforts in urban areas to reduce nutrient pollution in waterways. During stream restoration in urban ecosystems, trees are often removed from riparian zones to accommodate channel modification and heavy equipment access. This study assessed the impact of tree removal on riparian groundwater quality over space and time. Groundwater and stream water samples collected over 2 years across 5 study sites in Washington DC and Baltimore, ages 5, 10, and 20 years post-restoration, and analyzed for a suite of nutrients and heavy metals. Results showed increased concentrations of some nutrients and carbon in riparian groundwater for at least 5 years following tree removal but then declined after 20 year of recovery. Riparian zones appeared to act as sources or sinks of bioreactive elements depending on time since tree removal. This study showed that riparian tree removal can lead to significant groundwater quality impacts. However, groundwater quality improved 10-20 years after restoration. These effects of tree removal should be considered in cost-benefit analyses of restoration projects and where possible mature trees and soil profiles should be conserved.

Description:

Riparian zones are a vital interface between land and stream and are often the focus of stream restoration efforts in urban areas to reduce nutrient pollution in waterways. Restoring degraded stream channels often includes major physical alteration of the riparian zone to reshape streambank topography leading to the removal of mature trees. This study assessed the impact of tree removal on riparian groundwater quality over space and time. Twenty-nine wells were installed across 5 sites in watersheds of the Washington D.C. and Baltimore metropolitan areas in Maryland. Study sites encompassed a range in restoration ages (5, 10 and 20 years) as well as unrestored comparisons. Groundwater wells were installed as transects of 3 perpendicular to the stream channel to estimate nutrient uptake along groundwater flow paths. Well and stream water samples collected over a 2-year period (2018-2019) were analyzed for concentrations of dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), total dissolved nitrogen (TDN), and dissolved components of boron (B), calcium (Ca), copper (Cu), iron (Fe), potassium (K), magnesium (Mg), manganese (Mn), sodium (Na), and sulfur (S). Results showed some interesting patterns such as: (1) significantly increased mean concentrations of some nutrients and carbon in riparian groundwater for at least 5 years following tree removal then subsequent decline with recovery; (2) maximum TDN, DOC, and S concentrations at 5-year cut sites (20.5, 51.92, and 43.8 mg/L respectively) were higher than maximum TDN, DOC, and S concentrations at nearby comparison uncut sites (2.65, 18.53, and 14.1 mg/L respectively); (3) decreasing linear trends in concentrations of TDN, K and S during a 2 year shift from wet to dry conditions (p-value < 0.0001); (4) strong linear relationships between DOC (organic matter) and plant nutrients across sites suggesting the importance of plant uptake and biomass as sources and sinks of nutrients (p<0.05); (5) increasing concentrations along hydrologic flow paths from uplands to streams in riparian zones where trees were recently cut, and opposite patterns where trees were not cut. Riparian zones appeared to act as sources or sinks of bioreactive elements based on tree removal. Mean TDN, DOC, and S, concentrations decreased by 78.6%, 12.3%, and 19.3% respectively through uncut riparian zones, but increased by 516.9%, 199.7%, and 34.5% respectively through the 5-year cut transects. In contrast, concentrations of elements that are nonessential plant nutrients (e.g., Na and trace metals) did not share similar spatial or temporal patterns with the most bioreactive elements. Like other studies, results from this study showed that riparian tree removal can disturb multiple chemical constituents for the first few years after construction leading to significant groundwater quality impacts. However, this study also observed ecosystem recovery and an improvement in groundwater quality by 10-20 years after restoration. These effects of tree removal should be considered in cost-benefit analyses of restoration projects and where possible mature trees and soil profiles should be conserved. Overall, a more holistic understanding of the effects of riparian tree removal on groundwater quality can inform strategies for minimizing unintended negative consequences of stream restoration.

URLs/Downloads:

Record Details: