Geology topics

Joanna K. York

Publications and source records attributed to Joanna K. York.

3 recordsLinked to original sources

USGS2021

Oxygen-controlled recirculating seepage meter reveals extent of nitrogen transformation in discharging coastal groundwater at the aquifer–estuary interface

Nutrient loads delivered to estuaries via submarine groundwater discharge (SGD) play an important role in the nitrogen (N) budget and eutrophication status. However, accurate and reliable quantification of the chemical flux across the final decimeters and centimeters at the sediment–estuary interface remains a challenge, because there is significant potential for biogeochemical alteration due to contrasting conditions in the coastal aquifer and surface sediment. Here, a novel, oxygen- and light-regulated ultrasonic seepage meter, and a standard seepage meter, were used to measure SGD and calculate N species fluxes across the sediment–estuary interface. Coupling the measurements to an endmember approach based on subsurface N concentrations and an assumption of conservative transport enabled estimation of the extent of transformation occurring in discharging groundwater within the benthic zone. Biogeochemical transformation within reactive estuarine surface sediment was a dominant driver in modifying the N flux carried upward by SGD, and resulted in a similar percentage of N removal (~ 42–52%) as did transformations occurring deeper within the coastal aquifer salinity mixing zone (~ 42–47%). Seasonal shifts in the relative importance of biogeochemical processes including denitrification, nitrification, dissimilatory nitrate reduction, and assimilation altered the composition of the flux to estuarine surface water, which was dominated by ammonium in June and by nitrate in August, despite the endmember-based observation that fixed N in discharging groundwater was strongly dominated by nitrate. This may have important ramifications for the ecology and management of estuaries, since past N loading estimates have generally assumed conservative transport from the nearshore aquifer to estuary.

Limnology and Oceanography↗

USGS2019

Phytoplankton community structure response to groundwater-borne nutrients in the inland bays, Delaware

To determine the impacts of groundwater-borne nutrients on phytoplankton biomass and community structure, we conducted a series of mesocosm experiments in the Inland Bays of Delaware. Four treatments were tested, including mesocosms coupled directly to submarine groundwater seepage, mesocosms with the addition of pumped submarine groundwater, mesocosms with the addition of phosphate, and control mesocosms with no nutrient addition. We measured chlorophyll a concentrations as a proxy for overall biomass and used genetic sequencing techniques to characterize the phytoplankton community structure. Groundwater carried a high N load to the estuary with NO 3 - up to 295 µmol l -1 and NH 4 + up to 55 µmol l -1 . As a result, treatment mesocosms had elevated NO 3 - and NH 4 + , while control mesocosms were relatively low in nutrients. In June, the highest chlorophyll a concentrations occurred in mesocosms attached to seepage meters after 3.5 d, with significant differences across all treatments. In August, groundwater-amended mesocosms reached the highest biomass concentrations, which peaked after 3 d. There were significant differences across all treatments, except control and phosphate-amended mesocosms which remained unchanged. Community sequence data showed that species assemblage was also impacted by availability of nutrients, with significant differences in community structure for mesocosms receiving nutrients vs. control mesocosms in both June and August experiments. Harmful algal species proliferated in high nutrient treatments, including Cylindrotheca closterium , Karlodinium veneficum , Nitzschia spp., and Heterocapsa spp. While the general relationship between nutrient supply and biomass production is well known, we demonstrate the role groundwater-borne nutrients and sediment processes play in shaping community structure in estuarine primary producers and in promoting harmful algal blooms.

Delaware↗

USGS2015

Suburban groundwater quality as influenced by turfgrass and septic sources, Delmarva Peninsula, USA

Suburban land use is expanding in many parts of the United States and there is a need to better understand the potential water-quality impacts of this change. This study characterized groundwater quality in a sandy, water-table aquifer influenced by suburban development and compared the results to known patterns in water chemistry associated with natural, background conditions and agricultural effects. Samples for nutrients, major ions, and isotopes of N and O in NO 3 − were collected in 2011 beneath turfgrass from 29 shallow wells (median depth 3.7 m) and from 18 deeper wells (median depth 16.9 m) in a long-term suburban development. Nitrate (as N) concentrations in groundwater beneath turfgrass were highly variable (0.02–22.3 mg L −1 ) with a median of 2.7 mg L −1 , which is higher than natural water chemistry (>0.4 mg L −1 ; Na + –Cl − –HCO 3 − water type), but significantly lower than concentrations beneath a nearby agricultural area (median 16.9 mg L −1 ; p < .0001). Dissolved Fe concentrations in shallow suburban groundwater, attributed to chelated Fe in turfgrass fertilizers, were significantly higher ( p < .005) than concentrations from the agricultural site, although a Ca 2+ –Mg 2+ –Cl − –NO 3 − water type was dominant in both areas. A Na + –Cl − –NO 3 − water type indicated a septic-system source for nitrate in deep suburban groundwater (0.06–6.0 mg L −1 ; median 1.5 mg L −1 ). Isotopic data indicated denitrification; however, geochemical techniques were more helpful in identifying nitrate sources. Results indicate that suburban expansion into agricultural areas may significantly decrease overall nitrate concentrations in groundwater, but excessive turfgrass fertilization could result in localized contamination.

Delaware↗