WaterSubject

Saline water from a storm surge can flow down storm-damaged submerged water supply wells and contaminate boreholes and surrounding aquifers. Using data from conventional purging techniques, aquifer test response analysis, chemical analysis, and regression analysis of chloride/silica (Cl/Si) ratio, equations were derived to estimate the volume of saline water intrusion into a well and a porous media aquifer, the volume of water needed to purge a well shortly following an intrusion event, and the volume of water needed after delay of several or more months, when the saline plume has expanded. Purging time required is a function of volume of water and pumping rate. The study site well is located within a shoreline community of Lake Pontchartrain, St. Tammany Parish, in southeastern Louisiana, United States, which was impacted by two hurricane storm surges and had neither been rehabilitated nor chlorinated prior to our study. Chemical analysis of water samples in fall 2005 and purging of well and aquifer in June 6, 2006, indicated saline water had intruded the well in 2005 and the well and aquifer in 2006. The volume of water needed to purge the study well was approximately 200 casing volumes, which is significantly greater than conventionally used during collection of water samples for water quality analyses.Saline water driven by storm surges can flow down submerged storm-damaged water supply wells and contaminate boreholes (well casings and filter packs), resulting in potential impact to surrounding coastal aquifers. Determination of the extent of contamination and remediation needed is a water quality concern for private/domestic water supply wells. However, it appears there has been little research to address this problem, possibly due to difficulties in accessing wells following major natural disasters such as hurricanes. Tropical and extra tropical storms, including hurricanes, cyclones, typhoons, and northeasters, generate storm surges, which impact thousands of miles of coast lines along the world’s oceans every year. Many of these coastal areas are low lying and positioned close enough to mean sea level to be flooded by smaller storms (e.g., category 1 and 2 hurricanes), which generate a storm surge of up to 1.6 to 2.4 m. Approximately 57,640 km² of land along the U.S. Atlantic and Gulf coasts lie at an elevation under 1.5 m, most of it in four states: Louisiana (24,720 km²), Florida (12,720 km²), North Carolina (5840 km²), and Texas (5180 km²). This area includes coastal wetlands as well as portions of major metropolitan areas with more than 1 million residents, such as Baltimore, Boston, Miami, New York, New Orleans, Philadelphia, Tampa, and Washington, which lie within 1.5 m of mean sea level or and hence are vulnerable to storm surges from even a category 2 (154 to 177 km/h) hurricane.

However, the issue of coastal flooding with the potential for physical damage to thousands of water supply wells in urban settings followed by inundation of salt water from storm surges is not solely a North American problem. In Bangladesh, an area of approximately 22,000 km² could flood by a surge of only 1.5 m, which would impact approximately 17 million people and possibly thousands of water supply wells. Worldwide, saline surges can cover thousands of square kilometers, impact millions of people, and damage potentially thousands of private/domestic water supply wells. The focus of this study is on one well, out of thousands of wells damaged, within the storm surge zones of Hurricanes Katrina and Rita (B.C. Hanson, November 15, 2005, Baton, Rouge, Louisiana, personal communication). Three questions considered are the following: (1) Was the study well impacted by Katrina and Rita? (2) Was the impact still evident after 9 months? and (3) What was the extent and intensity of the remaining impact?

Previous studies have considered the impact of hurricanes on ground water but most have focused on either water levels or fluxes within the aquifers. noted that in a coastal area, surface flow patterns can impact ground water chemistry because of close coupling between ground water chemistry and surface water chemistry. A study in North Carolina’s Outer Banks considered the impact of Hurricane Isabel on five wells within the shallow water table aquifer only in terms of impact on water levels without any consideration of water quality impact. Also in North Carolina, considered the impacts of Hurricane Emily on the Cape Hatteras Aquifer. The aquifer was directly recharged by saline water, which caused elevated concentrations of chloride for a 4-year period. also considered Hurricane Emily’s impact on the same aquifer. Unlike this study, however, did not sample private wells. Furthermore, the Cape Hatteras unconfined aquifer was not used heavily as a source of water along the shoreline. Another study by considered hurricanes in terms of a source of recharge water due to the large volumes of precipitation available for unconsolidated units and metamorphic rock in Baja California. considered precipitation of Hurricane Mitch as a water source used as a tracer, given the isotopic difference from typical ground water within the karst aquifers of the Yucatan region of Mexico. Precipitation from hurricanes can provide a significant portion of recharge for unconfined coastal aquifers and can cause a major impact on spring and stream discharge considered the impact on the discharge from springs up to 20 km inland due to hurricanes that influenced tides for an unconfined karst aquifer in Florida. Still other workers considered the impact of hurricanes on ground water chemistry of the unconfined Biscayne Aquifer in Florida and the Cape Hatteras Aquifer in North Carolina. It appears that only a study by was concerned with the issue of saline hurricane surges impacting ground water used for water supply. study was of a small island unconfined aquifer system.

However, many aquifers in coastal areas are confined: Upper Florida Aquifer from southeastern South Carolina to southern Alabama; coastal aquifers of Texas; Chickasawhay River Aquifer of southern Mississippi Upper Chesapeake Aquifer from Maryland to North Carolina; Magothy Aquifer in New York; 800-foot sand aquifer in New Jersey similar to aquifers along the Louisiana coast. The need for investigation of well and aquifer contamination becomes apparent when examining past studies, which have not generally considered this problem. By contrast, this study focuses, in particular, on the water quality of storm-damaged private/domestic wells impacted by saline intrusion, contaminating boreholes (well casings and filter packs) and surrounding aquifers. These private wells tap an aquifer system that is the sole source of potable water for thousands of individuals in the coastal area of Louisiana impacted by storm surges. The particular storm surges considered in this study for their impact on private/domestic water wells are Hurricanes Katrina and Rita, which impacted Louisiana in August and September 2005. The procedure described in this paper provides a method to estimate the volume of saline water intrusion into the well and aquifer and to estimate the volume of water needed to purge the well and aquifer as well as the purging time required.

Saline water from storm surges can flow down submerged, damaged boreholes (well casings and filter packs), contaminating wells and surrounding coastal aquifers. A purging test and associated analyses in 2005 and 2006 were performed on a study well located near Lake Pontchartrain in southeastern Louisiana, United States, a coastal area impacted by Hurricanes Katrina and Rita in 2005. All five chemical indicators suggest that salt water intruded the study well during the storm surges of Katrina and Rita. The TDS, SC, and Cl values for initial samples in 2005 were elevated far above typical values for pre-Katrina (e.g., Upper Ponchatoula Aquifer) ground water, as determined from earlier studies. The sensitive Ca/Mg and Cl/Si ratios were far different values from those of pre-Katrina ground water. In 2005, the Cl/Si ratio was approximately 80,000% larger and the Ca/Mg ratio was approximately 6% of the ratios for pre-Katrina ground water.

Even after 9 months, the impact of storm surges was observed in the Ca/Mg and Cl/Si ratios. Results from analyses using both parametric and nonparametric tests indicate there was an impact on the Ca/Mg and Cl/Si ratios compared to pre-Katrina results and on the Ca/Mg and Cl/Si ratios and ratio trends during the purging test. For the nine samples collected during the purging test, both Cl/Si and Ca/Mg ratios were significantly different from values of previous studies for pre-Katrina ground water samples collected from wells near the shore of Lake Pontchartrain. A confidence of difference greater than 99.5% was determined from both of the data sets of pre-Katrina values.

The Ca/Mg and Cl/Si ratios indicate that as the purging test progressed, a larger share of the water was characterized as Upper Ponchatoula ground water. Although the impact on ground water decreased significantly over 9 months, it was still clear that Katrina and Rita did impact ground water around the damaged study well tested. In samples taken after the purging test, the Cl/Si ratio approximately doubled. This could be a result of (1) a secondary slug passing through an imperfect seal within the annular space between the well and the borehole; (2) a seasonal effect where chemistry varies due to temperature; or (3) an impact from a nearby well.

The volume of water impacted after 9 months was significantly larger than the volume of water that initially surged into the well. This is probably a result of a combination of diffusion, advection, and dispersion, which spread out the initial slug of water that entered the aquifer, and ion exchange, which prolonged the time required to purge the well and aquifer. The resulting volume required for purging to presurge chemistry was approximately 200 casing volumes, which is significantly larger than the range of 1 to 20 casing volumes necessary for representative water quality . To prevent or minimize saline intrusion, the grouting of annular space between well casing and borehole as well as hardening the well system with steel casing is suggested.

The general method for deriving equations from purging test, aquifer test response analysis, chemical analysis, and regression analysis can be applied for any porous media aquifer within a coastal region to yield an estimate of volume of saline intrusion and volume of water required to be purged from well and aquifer. Time required for purging is calculated as the required volume of water divided by pumping rate.

A recent study indicates that approximately 1,055,000 km² shoreline areas can be flooded by a surge of only 1 m, and that more than 100 million people live within this area. Even if only 10% of these people depend on private/domestic wells, it would indicate that several million water wells are vulnerable to damage and intrusion from saline water surges. In addition, three future trends indicate that possibly even more land, people, and private/domestic water supply wells could be vulnerable to saline storm surges: (1) increase of population in coastal areas.