Direction and hydraulic gradient

If a published water table map is not available for the area, but several wells and springs are nearby, a contour map of the water table may be drawn. Plot on a topographic map (at an appropriate scale) a sufficient number of points of static levels of water wells, observation wells, and test pits. Include spot elevations of perennial streams, ponds, and lakes. Using an appropriate contour interval, contour the data points to produce a useful water table map. Record dates of observations to allow comparison over time, from season to season, or in areas of suspected water table fluctuations.

If information on water table depths is not available and the aquifer is controlled by primary porosity, such as alluvium and glacial outwash, sketch several lines perpendicular to the elevation contours in the area of interest. The pattern that develops will indicate general ground water flow directions. Ground water discharge areas occur where the lines converge, such as most valleys, perennial streams, and ponds. Recharge areas, such as hilltops and upland areas converge, occur where the lines diverge.

For planning purposes, the general ground water flow direction and hydraulic gradient of the water table is calculated using data from three wells located in any triangular arrangement in the same unconfined aquifer (Heath 1983). They may be observation wells, test holes, test pits, or water wells. Also, the elevation of a perennial pond or stream can serve as an observation point. The 8-step procedure for this planning method follows, and figure 7A-1 gives an example.

Step 1—Obtain a detailed topographic map of the site, such as a USGS quadrangle or a field survey map. Be sure the map has a north arrow.

Step 2—Plot the position of the proposed AWMS component and all springs, whether developed or undeveloped, and wells within at least a half-mile radius. If the existence of wells is unknown, assume every rural house or farm/ranch headquarters represents the location of a well. Black squares on USGS quadrangles symbolize houses.

(meters or feet above mean sea level or an arbitrary datum plane) throughout this exercise.

Step 4—Measure the distance between the wells having the highest and lowest water level elevations, and record on the map.

Step 5—Using the map, identify the well with the intermediate water table elevation (that is, neither the highest nor the lowest). Interpolate the position between the well with the highest head and the well with the lowest head where the head is equal to that in the intermediate well. Mark this point on the map. Measure the distance between this point and the well with the lowest water level.

Step 6—Draw a straight line between the intermediate well and the point identified in step 5. This line represents a segment of a water table contour along which the head is the equal to that in the intermediate well.

Step 7—Draw a line perpendicular (90°) from this contour to the lowest head well, and measure the distance. This line is parallel to the ground water flow direction. Using the north arrow as a guide, orient a protractor to measure the compass direction of the line. Express the orientation of the ground water flow direction in degrees azimuth (clockwise east from north).

Step 8—Subtract the head of the lowest well from that of the intermediate well. Divide the difference by the distance measured in step 7. The result is the hydraulic gradient.

Step 3—Select three wells not in a line, and measure the static (nonpumping) levels using a commercial water depth meter or a lead weight on a measuring tape. Record on the map the head (elevation of the water table) for each well. Use consistent units

Figure 7A-1 Determining direction of ground water flow and hydraulic gradient (from Heath 1983)

Figure 7A-1 Determining direction of ground water flow and hydraulic gradient (from Heath 1983)

Appendix 7B

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