Geology topics

Tyler B. Hampton

Publications and source records attributed to Tyler B. Hampton.

3 recordsLinked to original sources

USGS2019

Residence time controls on the fate of nitrogen in flow‐through lakebed sediments

For many glacial lakes with highly permeable sediments, water exchange rates control hydrologic residence times within the sediment‐water interface (SWI) and the removal of reactive compounds such as nitrate, a common pollutant in lakes and groundwater. Here we conducted a series of focused tracer injection experiments in the upper 20 cm of the naturally downwelling SWI in a flow‐through lake on Cape Cod, MA. We systematically varied residence time and reactant controls on nitrate processing, using isotopically labeled 15 N nitrate to monitor the effect of these changes on nitrate removal via denitrification. The addition of acetate, a labile carbon compound, triggered the lake SWI to switch from net production to net removal of nitrate. When acetate was combined with increased residence time created by controlled reductions in water flux, we observed a fivefold increase in nitrate removal, a 26‐fold increase in N 2 production, and a 42‐fold increase in N 2 O production. We demonstrate that water residence time is an important control on the fate of nitrate in these lake SWIs and illustrate that seasonal conditions that alter lake exchange rates and variability in lake carbon may predict dynamic nitrate removal across the SWI. Additionally, observed N 2 O production during the oxic pore water experiments paired with geophysical characterization of the sediment porosity revealed that the lake SWI has less mobile pores occupying upward of 50% of the total porosity volume, which function as reactive microzones for nitrate processing.

Journal of Geophysical Research: Biogeosciences↗

USGS2019

Multi-scale preferential flow processes in an urban streambed under variable hydraulic conditions

Spatially preferential flow processes occur at nested scales at the sediment-water interface (SWI), due in part to sediment heterogeneities, which may be enhanced in flashy urban streams with heavy road sand influence. However, several factors, including the flow-rate dependence of preferential hyporheic flow and discrete groundwater discharge zones are commonly overlooked in reach-scale models of groundwater/surface water exchange. Using a series of controlled-head tracer-injection experiments coupled with cm-scale geophysics within the highly reactive upper 30 cm of the hyporheic zone of an urban stream, we quantified the flow dependence of local less-mobile porosity volume, mass-transfer rate coefficient, and the resulting local residence time in the less-mobile pore space at three controlled downward fluid fluxes (0.8, 2, and 3 m/d). Experiments were performed in two adjacent streambed locations, representing different sediment bulk vertical permeability. Less-mobile porosity parameters were generally substantial and similar between the two streambed locations; though a more competent, thin, organic layer at ∼15 cm depth in one location strongly impacted tracer loading, flushing dynamics, and local residence times. Increased downward flux led to (1) a decrease in less-mobile porosity residence time in all experiments, and (2) an increase in less-mobile porosity fraction for most experiments. Additionally, at the larger stream reach-scale, surface electrodes for electrical resistivity measurement were installed along 22 m of the wetted stream channel. These surface electrode measurements were collected during a natural storm flow event, which revealed widespread, short-term, flushing (e.g. <3 h) of the hyporheic zone with stream water, followed by longer-term (e.g. >60 h) flushing of the SWI with riparian zone groundwater. Flow dependence of preferential hyporheic zone flowpaths, like in the controlled tracer experiments, was also observed in these reach-scale electrical resistivity tomography measurements. Our findings reveal that the spatial and temporal dependence of preferential flow processes create highly dynamic SWI conditions that will affect the physical and coupled biogeochemical functions of the SWI in urbanized, sand-impacted streams.

Journal of Hydrology↗

USGS2018

Direct observations of hydrologic exchange occurring with less‐mobile porosity and the development of anoxic microzones in sandy lakebed sediments

Quantifying coupled mobile/less‐mobile porosity dynamics is critical to the prediction of biogeochemical storage, release, and transformation processes in the zone where groundwater and surface water exchange. The recent development of fine‐scale geoelectrical monitoring paired with pore‐water sampling in groundwater systems enables direct characterization of hydrologic exchange between more‐ and less‐mobile porosity during tracer tests. We adapt this technique to sandy interface sediments at a groundwater flow‐through kettle lake. Tracer experiments were conducted within controlled‐head permeameters over a range of specified downward flow conditions over several days. Although the bed was predominantly composed of highly permeable sands and gravels, cobble inclusions created less‐mobile flow zones at the centimeter scale. Less‐mobile porosity fractions, residence times, and rates of exchange were inferred from paired bulk and fluid electrical conductivity data, without the need for inverse model calibration. The conservative solute experiments were paired with 15 NO 3 − and other reactive amendments, revealing anaerobic processes occurring at shallow sediment depths where pore‐water sampling indicated bulk‐oxic conditions. The average less‐mobile porosity residence times as evaluated with the geoelectrical method were on 1‐hr timescales, which appear to be biogeochemically important in the context of creating anoxic microzones within less‐mobile porosity of sandy interface sediments.

Water Resources Research↗