Browsing by Author "Alexander, Richard B."

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    How Hydrologic Connectivity Regulates Water Quality in River Corridors
    Harvey, Jud; Gomez-Velez, Jesus D.; Schmadel, Noah M.; Scott, Durelle T.; Boyer, Elizabeth W.; Alexander, Richard B.; Eng, Ken; Golden, Heather; Kettner, Albert; Konrad, Chris; Moore, Richard; Pizzuto, Jim; Schwarz, Greg; Soulsby, Chris; Choi, Jay (2019-04)
    Downstream flow in rivers is repeatedly delayed by hydrologic exchange with off-channel storage zones where biogeochemical processing occurs. We present a dimensionless metric that quantifies river connectivity as the balance between downstream flow and the exchange of water with the bed, banks, and floodplains. The degree of connectivity directly influences downstream water quality - too little connectivity limits the amount of river water exchanged and leads to biogeochemically inactive water storage, while too much connectivity limits the contact time with sediments for reactions to proceed. Using a metric of reaction significance based on river connectivity, we provide evidence that intermediate levels of connectivity, rather than the highest or lowest levels, are the most efficient in removing nitrogen from Northeastern United States' rivers. Intermediate connectivity balances the frequency, residence time, and contact volume with reactive sediments, which can maximize the reactive processing of dissolved contaminants and the protection of downstream water quality. Our simulations suggest denitrification dominantly occurs in riverbed hyporheic zones of streams and small rivers, whereas vertical turbulent mixing in contact with sediments dominates in mid-size to large rivers. The metrics of connectivity and reaction significance presented here can facilitate scientifically based prioritizations of river management strategies to protect the values and functions of river corridors.
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    Low threshold for nitrogen concentration saturation in headwaters increases regional and coastal delivery
    Schmadel, Noah M.; Harvey, Judson W.; Alexander, Richard B.; Boyer, Elizabeth W.; Schwarz, Gregory E.; Gomez-Velez, Jesus D.; Scott, Durelle T.; Konrad, Christopher P. (2020-04)
    River corridors store, convey, and process nutrients from terrestrial and upstream sources, regulating delivery from headwaters to estuaries. A consequence of chronic excess nitrogen loading, as supported by theory and field studies in specific watersheds, is saturation of the biogeochemically-mediated nitrogen removal processes that weakens the capacity of the river corridor to remove nitrogen. Regional nitrogen models typically assume that removal capacity exhibits first-order behavior, scaling positively and linearly with increasing concentration, which may bias the estimation of where and at what rate nitrogen is removed by river corridors. Here we estimate the nitrogen concentration saturation threshold and its effects on annual nitrogen export from the Northeastern United States, revealing an average 42% concentration-induced reduction in headwater removal capacity. The weakened capacity caused an average 10% increase in the predicted delivery of riverine nitrogen from urban and agricultural watersheds compared to estimates using first-order process assumptions. Our results suggest that nitrogen removal may fall below a first-order rate process as riverine concentration increases above a threshold of 0.5 mg N l(-1). Threshold behavior indicates that even modest mitigation of nitrogen concentration in river corridors above the threshold can cause a self-reinforcing boost to nitrogen removal.
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    Small Ponds in Headwater Catchments Are a Dominant Influence on Regional Nutrient and Sediment Budgets
    Schmadel, Noah M.; Harvey, Judson W.; Schwarz, Gregory E.; Alexander, Richard B.; Gomez-Velez, Jesus D.; Scott, Durelle T.; Ator, Scott W. (2019-08-28)
    Small ponds-farm ponds, detention ponds, or impoundments below 0.01 km(2)-serve important human needs throughout most large river basins. Yet the role of small ponds in regional nutrient and sediment budgets is essentially unknown, currently making it impossible to evaluate their management potential to achieve water quality objectives. Here we used new hydrography data sets and found that small ponds, depending on their spatial position within both their local catchments and the larger river network, can dominate the retention of nitrogen, phosphorus, and sediment compared to rivers, lakes, and reservoirs. Over 300,000 small ponds are collectively responsible for 34%, 69%, and 12% of the mean annual retention of nitrogen, phosphorus, and sediment in the Northeastern United States, respectively, with a dominant influence in headwater catchments (54%, 85%, and 50%, respectively). Small ponds play a critical role among the many aquatic features in long-term nutrient and sediment loading to downstream waters. Plain Language Summary Reservoirs created by river damming have extensive impacts on downstream water quality but are not necessarily the most important elements of a diverse aquatic landscape. Many more small ponds have been constructed to serve important human needs ranging from farm irrigation in agricultural areas to flood control and trapping of nutrients and fine sediment in urban areas. The number of human-influenced small ponds is projected to rise worldwide, yet their role in the delivery of nutrients and sediment from headwaters to oceans is currently unresolved. Here we used new data sets and found that small ponds are collectively responsible for trapping a substantial amount of the nutrients and sediment that are exported annually from headwaters. These findings support the need to jointly consider features such as urban detention ponds, farm ponds, and beaver ponds in managing headwaters to decrease long-term nutrient and sediment loading to downstream waters and sensitive coastal areas.
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    Thresholds of lake and reservoir connectivity in river networks control nitrogen removal
    Schmadel, Noah M.; Harvey, Judson W.; Alexander, Richard B.; Schwarz, Gregory E.; Moore, Richard B.; Eng, Ken; Gomez-Velez, Jesus D.; Boyer, Elizabeth W.; Scott, Durelle T. (Springer Nature, 2018-07-17)
    Lakes, reservoirs, and other ponded waters are ubiquitous features of the aquatic landscape, yet their cumulative role in nitrogen removal in large river basins is often unclear. Here we use predictive modeling, together with comprehensive river water quality, land use, and hydrography datasets, to examine and explain the influences of more than 18,000 ponded waters on nitrogen removal through river networks of the Northeastern United States. Thresholds in pond density where ponded waters become important features to regional nitrogen removal are identified and shown to vary according to a ponded waters’ relative size, network position, and degree of connectivity to the river network, which suggests worldwide importance of these new metrics. Consideration of the interacting physical and biological factors, along with thresholds in connectivity, reveal where, why, and how much ponded waters function differently than streams in removing nitrogen, what regional water quality outcomes may result, and in what capacity management strategies could most effectively achieve desired nitrogen loading reduction.