8
It is extremely important to identify and protect the lateral area of influence. In general, the
lateral area of influence in a confined aquifer is 4,000 times larger than the LAI in an unconfined
aquifer. If a contaminant is introduced within the lateral area of influence, it will reach the well
faster than other water replenishing the well. This is true because the slope of the water tale
steepens toward the well within the area of influence. Experience has even shown this to be a
factor in protecting wells utilizing confined aquifers because of improper or failed well
construction. Production wells have become contaminated by their radius of influence
intercepting at some distance a well with failed well construction. In one example, a failed well
was 1000 feet from the production well and was cased from the surface to the confining layer.
The failed well was adjacent to a contamination source and the contaminant entered the confined
aquifer via the failed casing.
CASE STUDY EXAMPLES OF HOW TO DETERMINE
THE LATERAL RADIUS OF INFLUENCE
Determining the extent of the lateral radius of influence is a relatively straightforward procedure.
The following case studies contain three types of determination methods. The direct
measurement method involves measuring the drawdown in an observation well piezometer or a
production well located outside the minimum setback zone of the production well. The direct
measurement method determines if the well where the measurements are taken is within the area
of influence. The second type of determination uses exiting information, and the Theis equation
or the volumetric flow equation. The Theis equation can be used to estimate the lateral radius of
influence of a well if aquifer constants are available (e.g., storativity, transmissivity, and
hydraulic conductivity) or can be determined. The Theis equation is especially helpful if the
observation well was located within the minimum setback zone, i.e., the direct measurement
method cannot be used. The volumetric flow equation is another method of estimating the radius
of influence and is used for wells that pump continuously, and is a cost-effective alternative
where aquifer constants are not available to use the Theis equation. The third method involves
the interpretation of pump test data from observation wells within the minimum zone using a
curve-matching technique and the Theis equation to determine the storativity and the
transmissivity of the aquifer. Once these values have been determined, the Theis equation is
utilized to determine the lateral radius of influence of the well. It should be noted that if the
observation well of piezometer was outside the minimum zone that the determination could have
been made by direct observation.
