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Water Quality Program ROUTINE SAMPLING District staff measure water levels, collect water samples, and perform water-quality analysis on existing and newly drilled wells within the District boundries. Field parameters, including pH, conductivity, dissolved oxygen, temperature, and turbidity is measured using a portable Horiba U-10 water-quality probe. District staff perform a variety of laboratory tests to measure the presence or absence of pathogenic bacteria, the number of colonies of indicator coliform bacteria, alkalinity, as well as the levels of nitrate, iron, chloride, fluoride, and sulfate using a Hach DR 2000 spectrophotometer. Additionally, six wells and two springs are equipped with continuous temperature and electrical conductivity probes to monitor changes in water quality over time. GRANT-RELATED SAMPLING In 1990, 1993, and 1994 the District received grant funding from the Texas Water Development Board (TWDB) to analyze groundwater samples for a comprehensive list of groundwater parameters including pesticides, dissolved metals, alkalinity, radionuclides, petroleum hydrocarbons, and organics. Additionally, the District has been involved in a joint groundwater study with the TWDB since 1998. Annual sampling is conducted for approximately 25 wells and springs for field parameters (pH, conductivity, dissolved oxygen, temperature, and turbidity), nutrients, alkalinity, and an extensive list of dissolved metals. A comprehensive list of groundwater constituents was also analyzed from 28 wells and 6 springs in 2001. This study was included in an EPA 319h grant administered by the TNRCC to study non-point pollution. The results of that study indicate that groundwater contaminant levels in most of the sampled wells and springs are low compared to EPA maximum contaminant levels. Nine parameters were detected at levels above TNRCC Surface Water Standards. Though these data provide a good base line of water quality data, samples were collected during one period of flow conditions. Contaminant concentrations could vary significantly under different flow conditions, for example during storm events. Additionally, lower levels of contaminants may be present in some samples that could not be detected due to limitations of laboratory Method Detection Levels. The District is planning on pursuing grant funding for more water quality projects. Below is a brief summary of the BS/EACD groundwater sampling procedures and protocols as followed for grant purposes and District guidelines. WATER QUALITY STUDIES: General Guidelines Water-quality data for the Barton Springs segment is particularly important since many land-use changes are occurring in this area that could impact the water quality of the Edwards Aquifer. Potential sources of contamination to the aquifer can include: private on-site septic systems and municipal sewage collection lines; underground storage tanks; petroleum pipelines; subdivision development, roadway construction, golf courses, and urban runoff. Currently, rapid urbanization and extensive highway construction is underway and more is being proposed over the sensitive Recharge Zone. Highway expansion may foster the development and construction of new subdivisions and shopping centers, and increase vehicular traffic. Groundwater-quality studies can help planners, managers, and scientists establish baseline conditions and determine potential impacts in the Barton Springs segment. Purpose for Groundwater Sampling The purpose of groundwater sampling is to address the following question: What spatial changes in major constituents, nutrients, and organic compounds exist across the study area? What differences in water quality characterize the rural areas, urban areas, springs, Recharge Zone and Artesian Zone? What variation in water quality occurring over seasonal cycles is evident from historical data? Methodology Site Selection In order to select sampling sites that would be most representative of the Barton Springs segment, several selection criteria were established for candidate sites. Wells that lie on preferred groundwater flow paths are potential candidates. These sites are representative of a larger part of the Barton Springs segment. Troughs in the potentiometric surface often indicate the approximate location of preferred groundwater-flow paths. High-capacity pumping wells are also preferred sampling sites since their cones of depression tend to draw from a wider area. Prior to sampling, surface geology, well log, well depth information, and existing water-quality data for each sampling well is reviewed. It is important to select only those sampling points which have not been compromised by contaminants entering the well bore, and are representative of the water within the Edwards Aquifer. Although accessibility for water-level measuring is not critical to the sampling process, wells chosen for sampling should have access for water-level-measuring equipment whenever possible. Water-Level Measurement Water-level measurements are needed to estimate the amount of water that must be purged from a well prior to sample collection, and to construct potentiometric surface maps, which indicate groundwater flow directions and gradients. If a well selected for sampling is to also serve as a water-level measuring point, it is important that the measurement be made prior to well purging. Bailing, purging, or pumping will lower the water level. Well Purging The primary goal of any groundwater sampling project is the collection of samples from the subject wells and springs which are representative of the aquifer being studied. In order to accomplish this, all stagnant or standing water should be removed from the well prior to sampling. Water standing in a non-pumping well has little, if any, vertical mixing, which could cause stratification of the water. Stagnant water may also contain foreign material introduced from the surface, resulting in a sample not representative of true aquifer water quality. As a general rule, the evacuation or removal of three to five casing or borehole volumes of water from a well is sufficient to purge the well of stagnant water and replace it with representative aquifer water. The method used for purging is to continuously pump the well, while monitoring produced water, until parameters such as temperature, conductivity, and pH stabilized. These measurements are repeated every 5 minutes until they become consistent. Temperature is considered consistent when two temperature readings, taken 5 minutes apart, are within 0.1 degree centigrade. The pH reading is considered consistent if two readings, taken 5 minutes apart, are within 0.2 units. The conductivity is considered consistent when two conductivity readings, taken 5 minutes apart, are within 10%. Quality Control The use of duplicates, equipment blanks, and trip blanks for monitoring field quality assurance/quality control (QA/QC) performance is analogous to the use of similar procedures by laboratories to monitor internal QC. The goal of field QC is to ensure that sample protocol is being followed and that situations leading to error are recognized before they can seriously affect the data. The use of field QC samples can help identify changes in samples that occurred during sample collection, handling, storage, transportation, and laboratory procedures. Sample Handling and Custody Requirements The goal of sample custody is to account for the sample from the moment the water is placed in a sample container until all analytical tests have been completed and any remaining sample is discarded. Proper sample custody is a joint effort of the sampling crew, the sample transporter, and the laboratory staff. The main documentation of proper sample custody for all events up to and including the arrival of the sample at the laboratory is the Chain of Custody (COC) form. Custodial responsibility for the COC form passes from the individual that performs the sampling, to the transporting agent(s), to the designated custodian at the laboratory where analysis will occur, and finally to any LCRA or designated agent that retrieves, archives, or disposes of any remaining post analysis sample. Data Analysis The data are analyzed for spatial correlations in specific parameters, for example: areas where elevated levels of sulfate, strontium, and fluoride may suggest leakage from the Glen Rose Formation, and areas where elevated sodium and chloride indicate some influence from the saline water zone. Groundwater samples are collected and analyzed from wells and springs during sampling events. The water-quality results can be compared to a number of standards including:
Generally, analyses of groundwater samples collected from wells and springs include field parameters (temperature, pH, conductivity, and dissolved oxygen); metals (total and dissolved); anions; cations; pesticides herbicides; PCBs; VOCs; SVOCs; and other basic water-quality parameters. The following is a discussion of some of the key water-quality parameters. Chloride Chloride is found in all natural waters, but chloride in groundwater is primarily associated with sedimentary rocks, especially evaporites. In groundwater, when chloride is the most dominant anion, sodium is often the predominant cation. Human influences can also impact the amount of chloride found in groundwater. Chlorine is used to purify drinking water by killing bacteria. Also, chorine is used in the production of herbicides, pesticides, drugs, dyes, metals, and plastic. However, leakage from the saline water zone accounts for most of the elevated chloride levels measured in Edwards waters (Hauwert and Vickers, 1994). Fluoride Fluoride is found in most natural waters, but concentrations are generally low. This mineral tends to be found in carbonate rocks, along with volcanic rocks or sedimentary rocks derived from volcanic rocks. The amount of calcium found in groundwater can sometimes create a balance with fluoride concentrations. In other words, higher fluoride concentrations tend to occur when the groundwater has lower calcium concentrations. Groundwater taken from the saline water zone and deeper Glen Rose Aquifer can be distinguished by fluoride concentrations greater than 0.5 mg/L (Hauwert and Vickers, 1994). Human activities also impact the amount of fluoride in groundwater such as the manufacturing and production of glass, steel, aluminum, pesticides and fertilizers. Sulfate The most extensive source for sulfate in groundwater is evaporitic sedimentary rocks. When sulfide minerals weather, the sulfur is oxidized to release sulfate ions into solution. Groundwater in semiarid regions tends to be comparatively high in dissolved solids and sulfate is a predominate anion in most of these regions. Sulfates tend to indicate older, trapped groundwater. Samples taken from the saline water zone and from deeper within the Glen Rose Aquifer can be distinguished by sulfate concentrations greater than 50 mg/L (Hauwert and Vickers, 1994). Human factors influencing the amount of sulfates found in groundwater include sewage, various industrial wastewaters, production of sulfuric acid, metals, fertilizers, fungicides, insecticides, batteries, and medicine. Nitrogen, Nitrate and Nitrite Nitrate nitrogen is commonly introduced to groundwater by decaying organic matter, human and animal wastes, and fertilizers. Nitrate is considered a nutrient because it encourages algal growth and growth of other organisms which typically produce undesirable tastes and odors in groundwater. The EPA set drinking water standards at 10 mg/L based on ratios between high nitrate levels and the development of methemoglobinemia, a deadly disease for infants. The amount of nitrate measured in groundwater is generally dependent on amounts of rainfall (Schepers and Martin, 1986). The nitrate anion (NO 3-2) is the most common ionic form of nitrogen detected in groundwater. The nitrite and ammonium ions tend to be unstable in groundwater and therefore are less likely to be present. Sodium Sodium is an abundant element generally derived from igneous, metamorphic, and sedimentary rocks, with the highest levels in clay minerals, halite, and other evaporates. In addition, the production of table salt, industrial, agricultural and medical products can introduce sodium into the environment. The higher levels of sodium found in groundwater samples from the Barton Springs segment are probably influenced by the saline water zone, the Glen Rose Formation, development in areas west of the Recharge Zone. Conductivity Conductivity is a measure of the capacity of water to conduct an electric current, and can vary with the concentration and degree of ionization of the constituents in the water (EPA, 1986). In general, conductivity represents the mineral content of the water. Conductivity can be influenced by the amount of total dissolved solids (TDS), which comprise inorganic salts (primarily calcium, magnesium, potassium, sodium, bicarbonates, chlorides and sulfates) and small amounts of organic matter that are dissolved in water. Other sources of TDS can include runoff from urban areas such as fertilizers and pesticides. Generally, the amount of dissolved solids present in water increases proportionally with its electrical conductivity. Concentrations of TDS in water vary considerably in different geological regions resulting from differences in the solubilities of minerals that make up the aquifer. Generally, conductivity, or mineralization of the water, increases from the Recharge Zone to the Artesian Zone. Intensive faulting in the Edwards Aquifer has created barriers for groundwater flow to the east resulting in higher conductivity values in this area. There are several noticeable trends in the change in conductivity in relation to fluctuations in water levels and rainfall. Generally, as water levels increase after major rain events, conductivity tends to decrease. This suggests that rainwater, which is less mineralized, recharges the aquifer and can dilute high concentrations of organics, carbonic acid, nutrients and other ions that exist in the aquifer system, which eventually results in lower conductivity values. A drop in conductivity after a major rain event on may represent significant hydraulic connectivity between the surface and the well at a speicific monitoring site. Another trend is that conductivity and water level increase after a rain event, which may be the result of water recharging the aquifer and moving groundwater from low permeability portions of the aquifer into areas with higher TDS into fractures and conduits that flow towards the well. These two types of responses to a single rain event reveal the complexity of the aquifer system. Rainwater can enter the system through several different pathways, either directly through recharge features or by diffuse routes, which both can influence conductivity within a well. Conclusions Water-quality data are collected for the Barton Springs segment to evaluate current aquifer conditions. These data can be compared to data that have previously been collected in the study area. The additional knowledge gained by water-qualtiy studies is of significant importance to policy makers, planners, regulators, scientists, and resource managers to protect groundwater quality in the Barton Springs segment, to enhance the quantity of groundwater available for extraction, and to maintain springflow during times of drought. Annual sampling and analysis of groundwater is needed to provide a timely warning of serious increases in contaminant levels that can impact those that rely on the aquifer for drinking water and that can threaten aquatic life. |
