Tree Trade-Offs in Stream Restoration: Impacts on Riparian Groundwater Quality.

Riparian zones are a vital interface between land and stream and are often the focus of stream restoration efforts to reduce nutrient pollution in waterways. Restoration of degraded stream channels often requires the removal of mature trees during major physical alteration of the riparian zone to reshape streambank topography. We 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, Maryland, USA metropolitan areas. Study sites encompassed a chronosequence of 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. Groundwater 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 calcium (Ca), potassium (K), magnesium (Mg), sodium (Na), sulfur (S) and other elements. Results showed some interesting patterns such as: (1) elevated concentrations of some nutrients and carbon in riparian groundwater of recently restored (5 year) sites; (2) decreasing linear trends in concentrations of TDN, K and S in groundwater during a 2 year shift from wet to dry conditions; (3) linear relationships between DOC (organic matter) and plant nutrients in groundwater suggesting the importance of plant uptake and biomass as sources and sinks of nutrients; (4) increasing concentrations in groundwater 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. Ecosystem recovery and an improvement in groundwater quality appeared to be achieved by 10-20 years after restoration. A better understanding of the effects of riparian tree removal on groundwater quality can inform strategies for minimizing unintended effects of stream restoration on groundwater chemistry.

Stream and floodplain restoration impacts riparian zone hydrology of agricultural streams.

While the influence of stream restoration on vertical and lateral hyporheic exchange has been documented, impacts on broader riparian zone hydrology have not been thoroughly investigated. We quantified riparian water table dynamics, hydraulic gradient, and groundwater flow paths and fluxes across a range of hydrologic conditions following natural channel design restoration (riparian regrading, in-channel cross-vane structure installation). Water table measurements were collected at least once per season for 2.5 years to capture water levels during baseflow conditions from networks of wells and piezometers at sites with different stream morphology (created riffle-cross-vane-scour pool complexes versus natural pools and riffles), restoration status (agricultural restored, unrestored, forested reference), and riparian characteristics (slope, soils, topography) in North Carolina, USA. The regraded riparian zone had higher near-stream water tables (< 0.5 m below ground surface) than the unrestored site. Riffle-cross-vane complexes induced a zone of low hydraulic gradient that spanned 30-40% of the riparian area, similar to groundwater dynamics near beaver dams. This effect persisted regardless of hydrologic condition (wet or dry) or scour pool status (functioning or filled-in). Riffles also promoted a lower near-stream hydraulic gradient at the forested site. Conversely, the influence of stream features on riparian groundwater was minimal at the unrestored site, where groundwater fluxes were controlled by hillslope inputs and riparian geomorphology. Overall, restoration enhanced stream-riparian zone hydrologic interaction beyond the immediate hyporheic zone. Our work stresses that cross-vanes, even when partially buried by sediments post-restoration, impact whole floodplain hydrology in a more significant way than shown by prior stream restoration studies.

A new approach to generalizing riparian water and air quality function across regions.

There is growing interest in generalizing the impact of hydrogeomorphology and weather variables on riparian functions. Here, we used RZ-TRADEOFF to estimate nitrogen, phosphorus, water table (WT) depth, and greenhouse gas (GHG: N2O, CO2, CH4) functions for 80 riparian zones typical of the North American Midwest, Northeast (including Southern Ontario, Canada), and Mid-Atlantic. Sensitivity to weather perturbations was calculated for temperature and precipitation-dependent functions (CO2, phosphate concentration, and water table), and multivariate statistical analysis on model outputs was conducted to determine trade-offs between riparian functions. Mean model estimates were 93.10 cm for WT depth, 8.45 mg N L-1 for field edge nitrate concentration, 51.57% for nitrate removal, 0.45 mg PO43- L-1 for field edge phosphate concentration, 1.5% for subsurface phosphate removal, 91.24% for total overland phosphorus removal, 0.51 mg N m-2 day-1 for N2O flux, 5.5 g C m-2 day-1 for CO2 fluxes, and - 0.41 mg C m-2 day-1 and 621.51 mg C m-2 day-1 for CH4 fluxes in non-peat sites and peat sites, respectively. Sites in colder climates were most sensitive to weather perturbations for CO2, sites with deep water tables estimates had the highest sensitivity for WT, and sites in warm climates and/or with deep confining layers had the lowest sensitivity for phosphate concentration. Slope, confining layer depth, and temperature were the primary characteristics influencing similarities and trade-offs between sites. This research contributes to understanding how to optimize riparian restoration and protection in watersheds based on both water (nitrogen, phosphorus) and air quality (GHG) goals.

Denitrification along the Stream-Riparian Continuum in Restored and Unrestored Agricultural Streams.

Although floodplain restoration may improve stream-riparian connectivity and increase channel stability through cross-vane installation, stream bank regrading, and buffer revegetation, the impact of these geomorphic alterations on denitrification across the aquatic-terrestrial interface is poorly understood. Seasonal denitrification enzyme activity (DEA) was compared in stream-riparian ecotones of four streams with varying hydrologic connection, riparian vegetation composition, and agricultural influence. Riparian and stream DEA was generally higher in sites adjacent to agricultural fields due to proximity to nitrate source. Mean DEA was higher in the dormant season (riparian: 928 ± 116 ng N g dry mass [DM] h, stream: 108 ± 149 ng N g DM h) than in the growing season (riparian: 355 ± 55 ng N g DM h, stream: 45 ± 40 ng N g DM h) and was influenced by antecedent precipitation, soil texture, and landscape characteristics (land use, vegetation type, topography) in multivariate models. Hot spots of denitrification occurred at the confluence of fine soil textures with high organic matter and moisture, often at low-lying areas of the landscape. At the restored site, we observed enhanced denitrification in the reconnected floodplain but decreased stream denitrification capacity in pools. Across all sites, streams had lower DEA than riparian zones, stressing the importance of restoration approaches designed to conserve riparian areas and maintain hydrologic connectivity. Easily obtainable data including precipitation, adjacent land use, vegetation, and stream and floodplain geomorphology reasonably predicted denitrification potential compared with more complex models, showing potential for application in water resources and nitrogen management.

Changes in riparian hydrology and biogeochemistry following storm events at a restored agricultural stream.

Quantifying changes in riparian biogeochemistry following rainfall events is critical for watershed management. Following storms, changes in riparian hydrology can lead to high rates of nutrient processing and export and greenhouse gas (GHG) release. We assessed shifts in hydrology and biogeochemistry 24 and 72 hours post-rainfall following storms of three different magnitudes in an agricultural riparian zone influenced by stream restoration in the Piedmont region of North Carolina, USA. Post-storm changes in water table height, soil moisture, groundwater flow, and lateral hydraulic gradient were related to biogeochemical processing. Though near-field nitrate (NO3-) concentrations were elevated (median: 13 mg nitrogen (N) L-1 across storms), substantial riparian NO3- removal occurred (89-96%). High N removal throughout the study occurred concurrently with release of dissolved solutes (e.g., soluble reactive phosphorus [SRP]) and fluxes of gases (carbon dioxide [CO2], nitrous oxide [N2O], and methane [CH4]), based on storm timing, magnitude, and intensity. A high intensity, short duration storm of low magnitude lead to release of CO2 across the riparian zone and low SRP removal. A storm of intermediate duration/magnitude towards the beginning of the summer lead to mobilization of near-field NO3- and release of N2O in the upper riparian zone and SRP in the lower riparian zone. Finally, a larger storm of longer duration lead to pronounced near-stream release of CH4. Therefore, it is important to expand research of biogeochemical response to different types of storm events in restored riparian zones to better balance water quality goals with potential greenhouse gas emissions.

Twenty Years of Riparian Zone Research (1997-2017): Where to Next?

Riparian zones have been used for water quality management with respect to NO in subsurface flow and total P (TP), sediments, and pesticides in overland flow for decades. Only recently has the fate and transport of soluble reactive P (SRP), Hg, emerging contaminants, and greenhouse gas (GHG) fluxes (NO, CO, and CH) been examined in riparian zones. Overall, riparian zones are efficient at reducing emerging contaminants in subsurface flow and only function as hot spots of methylmercury production in the landscape when dominated by Hg-rich wet organic soils. However, riparian zones do not provide consistent benefits with respect to SRP removal or GHG emissions. Although most existing riparian models almost exclusively focus on NO removal, recent developments in riparian models demonstrate the potential for using easily accessible digital environmental datasets to simulate and scale up riparian functions beyond NO removal to include SRP, TP, and GHG dynamics. To further inform integrated watershed management efforts, more research should be conducted on how various practices, including stream restoration, subsurface drainage, two-stage ditches, beaver dam analogues, denitrification bioreactors and permeable reactive barriers, artificial wetlands, and short-rotation forestry crops affect riparian water and air quality functions. Riparian zone benefits should be discussed not only with respect to water and air quality, but also in terms of recreation, habitat for wildlife, and other ecosystem services. More research is needed to fully address potential water quality or air quality tradeoffs associated with riparian zone management in a multicontaminant-multiuse landscape context.

The impact of flooding on aquatic ecosystem services.

Flooding is a major disturbance that impacts aquatic ecosystems and the ecosystem services that they provide. Predicted increases in global flood risk due to land use change and water cycle intensification will likely only increase the frequency and severity of these impacts. Extreme flooding events can cause loss of life and significant destruction to property and infrastructure, effects that are easily recognized and frequently reported in the media. However, flooding also has many other effects on people through freshwater aquatic ecosystem services, which often go unrecognized because they are less evident and can be difficult to evaluate. Here, we identify the effects that small magnitude frequently occurring floods (< 10-year recurrence interval) and extreme floods (> 100-year recurrence interval) have on ten aquatic ecosystem services through a systematic literature review. We focused on ecosystem services considered by the Millennium Ecosystem Assessment including: (1) supporting services (primary production, soil formation), (2) regulating services (water regulation, water quality, disease regulation, climate regulation), (3) provisioning services (drinking water, food supply), and (4) cultural services (aesthetic value, recreation and tourism). The literature search resulted in 117 studies and each of the ten ecosystem services was represented by an average of 12 ± 4 studies. Extreme floods resulted in losses in almost every ecosystem service considered in this study. However, small floods had neutral or positive effects on half of the ecosystem services we considered. For example, small floods led to increases in primary production, water regulation, and recreation and tourism. Decision-making that preserves small floods while reducing the impacts of extreme floods can increase ecosystem service provision and minimize losses.

Denitrification potential in relation to lithology in five headwater riparian zones.

The influence of riparian zone lithology on nitrate dynamics is poorly understood. We investigated vertical variations in potential denitrification activity in relation to the lithology and stratigraphy of five headwater riparian zones on glacial till and outwash landscapes in southern Ontario, Canada. Conductive coarse sand and gravel layers occurred in four of the five riparian areas. These layers were thin and did not extend to the field-riparian perimeter in some riparian zones, which limited their role as conduits for ground water flow. We found widespread organic-rich layers at depths ranging from 40 to 300 cm that resulted from natural floodplain processes and the burial of surface soils by rapid valley-bottom sedimentation after European settlement. The organic matter content of these layers varied considerably from 2 to 5% (relic channel deposit) to 5 to 21% (buried soils) and 30 to 62% (buried peat). Denitrification potential (DNP) was measured by the acetylene block method in sediment slurries amended with nitrate. The highest DNP rates were usually found in the top 0- to 15-cm surface soil layer in all riparian zones. However, a steep decline in DNP with depth was often absent and high DNP activity occurred in the deep organic-rich layers. Water table variations in 2000-2002 indicated that ground water only interacted frequently with riparian surface soils between late March and May, whereas subsurface organic layers that sustain considerable DNP were below the water table for most of the year. These results suggest that riparian zones with organic deposits at depth may effectively remove nitrate from ground water even when the water table does not interact with organic-rich surface soil horizons.

Mercury dynamics in groundwater across three distinct riparian zone types of the US Midwest.

Although the intense biogeochemical gradients present in riparian zones have the potential to affect mercury (Hg) cycling, Hg dynamics in riparian zones has received relatively little attention in the literature. Our study investigated groundwater filtered total mercury (THg) and methylmercury (MeHg) dynamics in three riparian zones with contrasting hydrogeomorphic (HGM) characteristics (till, alluvium, outwash) in the US Midwest. Despite high Hg deposition rates (>16 µg m(-2)) in the region, median THg (<1.05 ng L(-1)) and MeHg (<0.05 ng L(-1)) concentrations were low at the study sites. Methylmercury concentrations were significantly (p < 0.05) correlated to THg (R = 0.82), temperature (R = 0.55), and dissolved organic carbon (DOC) (R = 0.62). THg also correlated with groundwater DOC (R = 0.59). The proportion of MeHg in THg (%MeHg) was significantly correlated to temperature (R = 0.58) and MeHg (R = 0.50). Results suggest that HGM characteristics, the presence of tile drains, and the propensity for overbank flooding at a riparian site determined the extent to which stream water Hg concentrations influenced riparian groundwater Hg levels or vice versa. Differences in hydrogeomorphic characteristics between sites did not translate however in significant differences in groundwater MeHg or %MeHg. Overall, widespread Hg contamination in the most common riparian hydrogeomorphic types of the US Midwest is unlikely to be a major concern. However, for frequently flooded riparian zones located downstream from a potentially large source of Hg (e.g., concentrated urban development), Hg concentrations are likely to be higher than at other sites.