H53A-01
Land or Ocean? Tracing Major Submarine Groundwater Discharge Driving Forces
The influence of submarine groundwater discharge (SGD) on nutrient and trace element cycling in the coastal ocean has been increasingly recognized, but little information about its driving forces is available. The current definition of SGD includes submarine fresh groundwater discharge (FSGD - the terrestrial component) and recirculated saline groundwater discharge (RSGD - the marine component). While FSGD is driven by the hydraulic gradient, RSGD incorporates recirculated water caused by wave set up, tidally-driven oscillations, current-induced pressure gradients, and convection. Since these terrestrial and marine driving forces are usually superimposed, it is difficult to separate their relative contributions. We report quasi continuous (<1h time steps), one year-long observations of the groundwater tracers CH4 and 222Rn, and coastal groundwater levels at a coastal site on the Gulf of Mexico. We have applied selected parts of this dataset to separate the relative contributions of the marine and terrestrial components of SGD. Even though 222Rn and CH4 have different geochemical behaviors, the significant correlations (p<0.01) between them indicate a common source for these constituents. The activities of 222Rn in the coastal water ranged from 1 to 15 dpm/L, usually showing higher values during the spring tide. Two processes may explain this pattern: enhanced tidal pumping force when tidal amplitudes are greater and/or longer tidal periods (occurs when the mixed semi-diurnal tide approaches diurnal character), which would provide more time for tracer regeneration within the sediments. We applied a mass balance based on 222Rn to estimate total SGD rates. We assume that the temporal variability of the total inventory of 222Rn in the water column is balanced by the difference between its total inputs (SGD and 226Ra decay) and total losses (atmospheric evasion and mixing with low concentration offshore waters). Radon-derived advection rates are highly variable, but typically around ~10 cm/day. These results were in good agreement with observations made during intensive experiments, when we also deployed seepage meters along a shore-normal transect. To derive the terrestrial component of SGD, we used continuous observations of groundwater levels in a 3-m deep, freshwater well located onshore, 15m away from the high tide mark. The local water table responds very quickly (on a scale of hours) to rainfall events. While immediately after rain events the water table drops relatively rapidly (5-8 cm/day), it drops at a slower rate of 0.5-2 cm/day after several hours. We assume that the observed rate of decay of the water table is equivalent to FSGD, after accounting for local porosity (<0.3) and neglecting evapotranspiration. The comparison of both approaches indicate that the terrestrial component accounts for <10% of total SGD in our study area. This is consistent with direct measurements of salinity of the water emanating from the sediments. Based on a simple mixing model of seawater and freshwater, we estimated that FSGD may range from 0 to 18%, in good agreement to our 222Rn model - hydrological approach. We thus conclude that marine forces are the dominant SGD drivers at this site.
H53A-02
LASTRIG -A Multiple Parametric Method of Assessment of Salinization Vulnerability of a Coastal Aquifer in Pennar Delta, India
Coastal populations are critically dependent upon the coastal aquifers for their freshwater requirements. Excessive withdrawal of groundwater leads to saline incursion and the consequent degradation of quality and quantity of freshwater. The paper describes a multiple parametric method of assessment of vulnerability of the coastal aquifer in Pennar delta, south India, in the context of the hydrogeological, biophysical, geochemical and socioeconomic environments of the delta. Seven parameters, forming the acronym LASTRIG viz. landuse, aquifer type, soil depth, groundwater table, rainfall, soil infiltration and geomorphology are made use in the assessment, and involve the use of remote sensing, GIS and modeling tools. The parameters are weighted, and a suitable ranking system has been designed to quantify the degree of vulnerability of the aquifer for salinization. It has been found that zones with high vulnerability index correlate well with zones of high TDS and chloride contents of groundwater. This observation thus validates the geochemical basis of the proposed LASTRIG system. The new system has been found useful in the management of the groundwater resources of the delta region. It has been made use of identify the aquifer segments which are in danger of being degraded, to enable the decision- makers to design counter measures to avoid further deterioration in water quality. Where the groundwater has already been rendered non-potable because of saline incursion, the LASTRIG index could be made use to identify possible use of that water for drinking by cattle, and for growing of salt-tolerant vegetables (e.g. beetroot and lettuce), and trees (e.g. casuarinas obese, Prosopis juliflora)
H53A-03
Remediation process of groundwater quality in Asian Tsunami affected Southern Sri Lanka- Case study in Kumbalgama area
Prior to the Tsunami wave occurred on 26th December 2004, groundwater in the southern coastal margin was non saline and used by the people for drinking and other domestic purposes. The preliminary results of the study revealed that the Electrical conductivity of well water in all wells situate in the Tsunami affected Zone are turned to be saline (EC in average increases from 300 µ Siemens per cm to around 5000 µ siemens /cm and above). Total dissolved solids of the well waters are around 2000 mg/L. Groundwater table behavior and quality have been studied from March 2005, in the southern coastal margin Sri Lanka, which situates in the tsunami affected zone (Lat 80.42. Long 5.97). 30 affected and non-affected shallow dug wells, by the Tsunami wave, situates in 7 km coastal strip, were selected for the study. The selected wells are sunk into the permeable quaternary sand deposits overlying Precambrian granite gneiss. The top quaternary sandy aquifer in the coastal margin of Weligama bay area is very permeable and hydro geological conditions are very favorable for salt-water intrusion. The study helped to prepare the hydrogeological, and the hydro-geo- chemical maps of the area. According to the atmospheric precipitation from January 2005 to June 2006, most of the months received more than 50 mm of rain except in Feb, Sept 2005 and Feb, April 2006. The maximum of 423 mm rainfall has received in November 2005. During the sampling period, January 2006 to June 2006 total rainfall received by the locality is 452 mm. But in the some affected wells groundwater quality was improved to 1500 to 2000 µ siemens /cm due to high atmospheric precipitation. According to groundwater map, Hydrogeological condition of the area is the on of the most affect to remediation of quality in groundwater.
H53A-04
Isotopic composition of continental and marine waters in Costa Chica, Guerrero, Mexico
The region interesting to the present hydrologically-oriented study is part of the Costa Chica, between the state line of Guerrero and Oaxaca, in southern Mexico, which includes the warm and subhumid lowlands of the 6136 km2 watershed of the Ometepec river. This perennial, although highly seasonal river, drains the southern flank of Sierra Madre del Sur, an elevated mountain range parallel to the Pacific Ocean shoreline, with peaks up to 3000 m asl, and at 98º 43' W; 16º 30' N, discharges into this ocean around 3 × 109 m3 of pristine water per year. So far, anthropic alterations of the whole environment are not immediately obvious. Its coastal wetlands have become a new international Ramsar Site. Our region of interest is a large scale unperturbed natural laboratory, waiting to be studied. The present study shows a several years ongoing survey of 64 paired measurements, (δ 18O, δ 2H) in permil versus Vienna SMOW, of the oxygen-18 and deuterium concentrations of the continental and marine waters of the study region. Rain water was not sampled. The objective is to show and personally discuss the sole "isotopic picture" in a threefold way: i) To test locally the meteoric line, the fractionation coefficients, and the altitude effect, all of them documented for central Mexico through other studies, and, also, to compare the isotope data available for other locations nearby; ii) To display the regularities we found in surface and ground waters, potentially useful studying its catchment isotope hydrology; and iii) To show marine water features that could be indicative of local continental discharges inside the sea, the ones that should be taken into account in future hydrological and ecological balances [in: Zektser IS, RG Dzhamalov, LG Everett (2007). Submarine Groundwater. CRC Press, Boca Raton. 446 pp.]