Biogeochemical reactions within intertidal zones of coastal aquifers have been shown to alter the concentrations of terrestrial solutes prior to their discharge to surface waters. In organic‐poor sandy aquifers, the input of marine organic matter from infiltrating seawater supports active biogeochemical reactions within the sediments. However, while the seasonality of surface water organic carbon concentrations (primary production) and groundwater mixing have been documented, there is limited understanding of the transience of various organic carbon pools (pore water particulate, dissolved, sedimentary) within the aquifer and how these relate to the location and magnitudes of biogeochemical reactions over time. To understand the relationship between changes in groundwater flow and the seasonal migration of geochemical patterns, beach pore water and sediment samples were collected and analyzed from six field sampling events spanning 2 years. While the seasonally dynamic patterns of aerobic respiration closely followed those of salinity, redox conditions and nutrient characteristics (distributions of N and P, denitrification rates) were unrelated to contemporaneous salinity patterns. This divergence was attributed to the spatial variations of reactive particulate organic carbon distributions, unrelated to salinity patterns, likely due to filtration, retardation, and immobilization dynamics during transport within the sediments. Results support a “carbon memory” effect within the beach, with the evolution and migration of reaction patterns relating to the distribution of these scattered carbon pools as more mobile solutes move over less mobile pools during changes in hydrologic conditions. This holds important implications for the prediction and quantification of biogeochemical reactions within beach systems.
The intertidal zone of beach aquifers hosts biogeochemical transformations of terrestrially derived nutrients that are mediated by reactive organic carbon from seawater infiltration. While dissolved organic carbon is often assumed the sole reactive organic carbon component, advected and entrapped particulate organic carbon (POC) is also capable of supporting chemical reactions. Retarded advection of POC relative to groundwater flow forms pools of reactive carbon within beach sediments that support biogeochemical reactions as dissolved solutes move across them due to transient groundwater flow. In this work, we simulate the contribution of POC to beach reactions and identify parameters that control its relative contribution using a groundwater flow model (SEAWAT) and reactive transport model (PHT3D). Results show transient contributions of POC to denitrification, as the spatial extent of the saline circulation cell varies over time due to changing hydrologic factors. A decrease in POC retardation and an increase in tidal amplitude during POC deposition resulted in POC expansion, which increased the relative contributions of POC to beach reactivity. Decreased hydraulic conductivity and increased tidal amplitude post‐deposition decreased the utilization of POC for denitrification by allowing the oxic, saline water to completely encompass the pool of POC. Results highlight that POC is an intermittently utilized source of carbon that displays complex spatial relationships with groundwater flow conditions and overall beach biogeochemistry. This work demonstrates that POC may be a periodically important but overlooked contributor to biogeochemical reactions in carbon‐poor beach aquifers.
more » « less- NSF-PAR ID:
- 10448259
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Water Resources Research
- Volume:
- 56
- Issue:
- 10
- ISSN:
- 0043-1397
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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