Water Table Measurements

A variety of simple, accurate, and professional tools are available to measure water table depths to an accuracy of 0.2 cm. These instruments consist of a wire with a metal tip that is lowered and upon hitting the water an electrical circuit closes, indicating the water table (Fig. 4.2). The wire is marked for depth readings. The described mode of water table measurement can be performed only in an open, unequipped well. A pumped well can be specially equipped for water table measurements, and this is of interest as it reveals the local dynamics of the lowering of the water table due to abstraction (section 4.6). 

The water table is dynamic in most systems. It changes in response to rain events, flood events, snowmelt, recharge, and pumping. To decipher the interplay of these ongoing processes, periodic water table measurements are essential. Historical data are in many cases available from local water authorities, which conduct routine water table measurements. Measurement of the depth of the water table at the time of each sample collection is essential in order to couple the chemical and isotopic results with the hydrological data. 

Repeated water table measurements in a well may be presented on a hydrograph, as a function of time. In the example given in Fig. 4.7, the low water table may be interpreted as reflecting lack of recharge in the winter, and the rise may reflect snowmelt recharge followed by summer rains. Countless combinations of hydrograph shapes and modes of interpretation are possible. Knowledge of local precipitation and climate is needed for proper interpretation. Rapid response to rain or flood events may indicate... 

Water table fluctuations are occasionally accompanied by measurable variations in water temperature or composition, providing crucial information on mixing of different water types. Water table measurements are an important tool in tracing recharge. Three cases, reported by Winslow et al. (1965) are discussed in the following sections. 

A detailed hydrograph, based on frequent water table measurements at an observation well at the Saratoga National Historic Park, New York, is given in Fig. 4.8, along with the local precipitation (Winslow et ah, 1965). The authors offered the following interpretation ... 

The operation of the nitronium ion in these media was later proved conclusively. "- The rates of nitration of 2-phenylethanesulphonate anion ([Aromatic] < c. 0-5 mol l i), toluene-(U-sulphonate anion, p-nitrophenol, A(-methyl-2,4-dinitroaniline and A(-methyl-iV,2,4-trinitro-aniline in aqueous solutions of nitric acid depend on the first power of the concentration of the aromatic. The dependence on acidity of the rate of 0-exchange between nitric acid and water was measured, " and formal first-order rate constants for oxygen exchange were defined by dividing the rates of exchange by the concentration of water. Comparison of these constants with the corresponding results for the reactions of the aromatic compounds yielded the scale of relative reactivities sho-wn in table 2.1. 

A variety of measures must be taken to assure safe and continued operation. Because the natural water table is higher than the coal seams, or the seams are natural aquifers, it is necessary to pump water out of the pit or to drill weUs around the mine and pump to reduce the water table. The Rheinische Braunkohlenwerke (Rheinbraun) pumps water at a rate of 1-1.2 x 10 m /yr. Part of this water is processed to provide drinking water for Neuss and Dusseldorf. The tendency of lignite to ignite spontaneously requires care in the amount of face that is exposed, especially in naturally dry, hot, windy climates. 

An important aspect of economic consideration is the prevention of egg and egg product loss to the drain or the atmosphere, eg, a checked or cracked egg may be broken in the washer, and the contents go down the drain with the wash water. Other measurable losses during egg product production are Hsted in Table 5. 

These expressions may be rearranged to calculate the specific or molar heat capacity from the measured temperature rise caused by a known quantity of heat. The specific heat capacity of a dilute solution is normally taken to be the same as that of the pure solvent (which is commonly water). Table 6.2 lists the specific and molar heat capacities of sume common substances. 

For multi-pH liposome-water distribution measurements in 0.15 M NaCl or KCl solutions, the difference between the true pKj and pK (and between logPMBM and logPMBM) is about 1 log unit for bases and 2 log units for acids. Liposomes formed from phosphatidylcholine have a tendency to stabilize the charged drug more effectively than that in the octanol-water system [71]. Table 3.1 shows liposome-water examples of the relative pKj shifts. The average values cited in the examples are close the diff 1-2 approximation noted above. 

Installation of the pumping well is more time consuming than digging a trench. There is a lag period between the start of pumping, the formation of the depression cone, and containment of the plume. This limits its use as a rapid containment measure. The water table depression must be kept constant otherwise, if the water table is allowed to fluctuate, gasoline droplets may adhere to soil particles and get trapped below the water table, especially when the depth of the cone of depression gets lower. 

The results obtained by various calibrations in the determination of nickel and copper are shown in Tables 1.2 and 1.3. Table 1.4 gives the differences between sampling devices for copper, as determined by each participant, when these are significant at the 95% and 90% levels of confidence. Only the results of participants that had acceptable analytical performance, as measured by precision and agreement with contemporary consensus values for deep North Atlantic waters (Table 1.5), were used for drawing conclusions. 

Field measurements included pH, Eh, EC, temperature, depth of water table and depth of sample collection. Separate, field preserved sub-samples were collected for cation (ICP-MS/OES), anion (IC), alkalinity (titration), DOC, P04 and Au and PGE analysis (using carbon sorption). 

In place of distance measurements, concentration can be used to determine the age of a plume. Assuming that the plume is derived from a single slug-type release of a contaminant which is only partially soluble in groundwater, and not present as an NAPL, then the date upon which the contaminant enters the water table or aquifer is equivalent to that point in time when the concentration of the contaminant approaches its solubility limit ... 

It is assumed that the contaminant enters the water table or aquifer at a concentration near its solubility limit, although there is no practical means to verify this. This method is more favorable when the release occurred as a single, short-term episode. A long-term release from a continuing source would result in a date that more closely represents the last date upon which the contaminant entered the aquifer at or near its solubility limit. Should the contaminant enter the aquifer below its solubility limit, then a date earlier than the actual event would result. Conversely, should the contaminant enter the aquifer as NAPL for a period of time, a date in which all the NAPL dissolved in groundwater would result. If NAPL was present when measurements were obtained, then the zone of highest concentration would... 

The measured or apparent hydrocarbon thickness is not only dependent on the capillary fringe but also on the actual hydrocarbon thickness in the formation (Figure 6.6b). In areas of relatively thin LNAPL accumulations, the error between the apparent well thickness and actual formation thickness can be more pronounced than in areas of thicker accumulations. The larger error reflects the relative difference between the thin layer of LNAPL in the formation and the height it is perched above the water table. The perched height is constant for thick and thin accumulations however, a thick accumulation can depress and even destroy the capillary fringe as illustrated in Figure 5.1. 

The thickness measured in a monitoring well with LNAPL product situated on a perched layer at some elevation above the water table can produce an even larger... 

Additionally, vertical fluctuations in the water table due to recovery operations or seasonal variation in precipitation have a direct effect upon the apparent or measured LNAPL thickness (Figure 6.6d). As the water table elevation declines gradually due to seasonal variations, for example, an exaggerated apparent thickness occurs, reflecting the additional hydrocarbon that accumulated in the monitoring well. The same is true for an area undergoing recovery operations where the... 

Since the water table as measured in the well is depressed by the weight of the hydrocarbon, a corrected depth to water is calculated ... 

The recoverability of hydrocarbon from the subsurface refers to the amount of mobile hydrocarbon available. Hydrocarbon that is retained in the unsaturated zone is not typically recoverable by conventional means. Additional amounts of hydrocarbon that are unrecoverable by conventional methods include the immobile hydrocarbons associated with the water table capillary zone. Residual hydrocarbon is pellicular or insular, and is retained in the aquifer matrix. With respect to recoverability, residual hydrocarbon entrapment can result in volume estimate discrepancies as well as decreases in recovery efficiency. With increasing water saturation, such as when the water table rises via recharge or product removal, hydrocarbons essentially become occluded by a continuous water phase. This results in a reduction of LNAPL and product thickness as measured in the well at constant volume. When water saturation is decreased by lowering the water table (as during recovery operations), trapped hydrocarbons can remobilize, leading to increased recoverability. 

This example demonstrates that it is possible to make reasonably accurate predictions of production after the pattern of production is established. However, caution must be exercised by the professional when developing these data. The example cited occurred during a period when the water table at the facility was within the normal ranges. If the water table had risen, or fallen substantially (for any reason), the pattern of production would have changed, and the calculations would not be considered reliable. Estimation of reserves determined by these methods can be fairly reliable if the data used are based on adequate and regular measurements, and are applied with a reasonable measure of professional judgment. 

A common consideration is the presence of water vapor, H20(g). Water generates a vapor pressure, which varies with the temperature. Daltons law is used in these cases to adjust the pressure of a gas sample for the presence of water vapor. The total pressure (normally atmospheric pressure) is the pressure of the gas or gases being collected and the water vapor. When the pressure of an individual gas is needed, the vapor pressure of water is subtracted from the total pressure. Finding the vapor pressure of water requires measuring the temperature and using a table showing vapor pressure of water versus temperature. 

TABLES. Measured and Correlated pH of Water-Ammonia Mixtures with 0.491 Mole of H S/Mole of NH, 25°C... 

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