Abstract
Submarine groundwater discharge (SGD) to the coastal environment along the eastern Yucatan Peninsula, Quintana Roo, Mexico was investigated using a combination of tracer mass balances and analytical solutions. Two distinct submarine groundwater sources including water from the unconfined surficial aquifer discharging at the beach face and water from a deeper aquifer discharging nearshore through submarine springs (ojos) were identified. The groundwater of nearshore ojos was saline and significantly enriched in short-lived radium isotopes (223Ra, 224Ra) relative to the unconfined aquifer beach face groundwater. We estimated SGD from ojos using 223Ra and used a salinity mass balance to estimate the freshwater discharge at the beach face. Analytical calculations were also used to estimate wave set-up and tidally driven saline seepage into the surf zone and were compared to the salinity-based freshwater discharge estimates. Results suggest that average SGD from ojos along the Yucatan Peninsula Caribbean coast is on the order of 308m3d−1m−1 and varies between sampling regions. Higher discharge was observed in the southern regions (568m3d−1m−1) compared to the north (48m3d−1m−1). Discharge at the beach face was in the range of 3.3–8.5m3d−1m−1 for freshwater and 2.7m3d−1m−1 for saline water based on the salinity mass balance and wave- and tidally-driven discharge, respectively. Although discharge from the ojos was larger in volume than discharge from the unconfined aquifer at the beach face, dissolved inorganic nitrogen (DIN) was significantly higher in beach groundwater; thus, discharge of this unconfined beach aquifer groundwater contributed significantly to total DIN loading to the coast. DIN fluxes were up to 9.9mold−1m−1 from ojos and 2.1mold−1m−1 from beach discharge and varied regionally along the 500km coastline sampled. These results demonstrate the importance of considering the beach zone as a significant nutrient source to coastal waters for future management strategies regarding nutrient loading to reef environments and coastal development. This study also identifies the importance of understanding the connectivity of submarine spring discharge to the nearshore coastal environment and the impact of inland anthropogenic activities may have on coastal health.
Highlights
Submarine groundwater discharge (SGD), the discharge of subterranean fresh water and recirculated seawater to the coastal zone, occurs throughout the world's coastlines and has been identified as an important source of nutrients to many coastal ecosystems (e.g. Corbett et al, 1999; de Sieyes et al, 2008; Knee et al, 2010; Paytan et al, 2006; Slomp and Van Cappellen, 2004)
We found beach groundwater 223Ra activities in the range of 0.1–2.7 dpm 100 LÀ1 and the highest 223Ra activity was measured in ojos (66.4 dpm 100 LÀ1) with a range of 0.2– 66.4 dpm 100 LÀ1. 224Ra activities showed similar trends with greater activities in ojos (0.6–51.9 dpm 100 LÀ1) than in the beach groundwater (0.4–14.7 dpm 100 LÀ1). 223Ra and 224Ra activities were lower in fresh beach groundwater samples compared to ojos and saline beach groundwater, except at Xcalak where fresh beach groundwater activities were greater than saline beach groundwater activities (Table 2)
Since this study identified the southern region of the Yucatan Peninsula as having much greater discharge rates compared to the northern sites, we suggest that development and potential contamination in the southern aquifers may have a more negative impact on the marine coast, and any coastal or inland development plans should consider the regional hydrogeology in addition to other factors
Summary
Submarine groundwater discharge (SGD), the discharge of subterranean fresh water and recirculated seawater to the coastal zone, occurs throughout the world's coastlines and has been identified as an important source of nutrients to many coastal ecosystems (e.g. Corbett et al, 1999; de Sieyes et al, 2008; Knee et al, 2010; Paytan et al, 2006; Slomp and Van Cappellen, 2004). Submarine groundwater discharge (SGD), the discharge of subterranean fresh water and recirculated seawater to the coastal zone, occurs throughout the world's coastlines and has been identified as an important source of nutrients to many coastal ecosystems Submarine spring discharge in coastal karst systems can be fresh or saline depending on the geology, hydraulic head, rainfall, and extent of subsurface mixing with intruded seawater (Bonacci and Roje-Bonacci, 1997; Fleury et al, 2007). There is a need to better understand how anthropogenic activity and groundwater delivery in areas with karst geology are linked to land use and pollutant transport that impact local coastal ecosystems
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