Groundwater Contribution to Coastal Nutrient Loading Along the Gulf Shores of Alabama
Abstract
Recently it has been recognized that submarine groundwater discharge (SGD) may be one of the principal mechanisms for delivering nutrients to surface water bodies, resulting in eutrophication of many near-shore coastal areas throughout the world. A groundwater flow and contaminant transport model was integrated with field measurements of naturally occurring radiogenic tracers and nutrient sampling to determine localized groundwater flow paths and nutrient flux to the near-shore coastal system. Specifically, a three-dimensional (3-D) numerical model coupling density-dependent groundwater flow codes (MODFLOW/SEAWAT) with a solute transport code (MT3DMS) was used to simulate the transport of nitrate and sulfate through the groundwater system to the coast. Borehole data from 35 wells, including formation data and electric logs were used to construct the subsurface lithology. Recently determined local and regional recharge rates, nutrient sinks and sources, and groundwater pumping rates were incorporated into the model. A final hydraulic head calibration variance of 0.076 meters was obtained over a sixteen year period utilizing nonlinear numerical Parameter Estimation (PEST) software. A total of 100 nutrient samples were taken in duplicate from 32 wells within the study area and analyzed for chloride, nitrate, and sulfate to assess contaminant source zones, estimate aquifer nutrient fluxes, and characterize the freshwater/saltwater interface. Five independent 222Radon time-series surveys were conducted across Lake Shelby and along adjacent near-shore boundaries. Groundwater seepage was calculated through a mass-balance model, where 222Radon inventories were converted to fluxes accounting for losses from atmospheric evasion, tidal fluxes, and mixing from marine water. This study expands upon previous research integrating the results of a 3-D groundwater flow and transport model with direct tracer measurements to more accurately determine SGD pathways, contributions of SGD, and nutrient flux to the near-shore freshwater/saltwater boundary zone and the Gulf of Mexico. This research demonstrates the success of a combined numerical modeling and field tracers approach for evaluating the contribution and significance of SGD on nutrient loading to coastal near-shore environments.
- Publication:
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AGU Spring Meeting Abstracts
- Pub Date:
- May 2013
- Bibcode:
- 2013AGUSM.H33A..02E
- Keywords:
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- 4536 OCEANOGRAPHY: PHYSICAL / Hydrography and tracers;
- 1847 HYDROLOGY / Modeling;
- 1872 HYDROLOGY / Time series analysis