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1 � APPLICATION OF SLUG TEST TO CONFINED AQUIFERS Theoretically, the slug test (Bouwer and Rice, 1976) applies to aquifers K here the upper bound- ! 1 ary is a plane source (rising water-level test)or sink i (falling water-level test), as in an unconfined aquifer However, because most of the head difference y between the static water table and the Y water level in the well is dissipated in the vicinity of the well around the screen or perforated section, 1 1 the method should also be applicable to situations 1 where the upper boundary of the aquifer is an 1 impermeable or semipermeable plane, i e , an i 1 L impermeable or semipermeable upper confining layer Thus the slug test should also give reasonable l values for K in confined, semiconfined, or stratified i aquifers Theoretically, the larger the distance ibetween the top of the screened or open section of the well and the upper confining layer (like Lw — 4 Fig 6 Schematic of addition of water (hatched section) to in Figure 1), the more accurate the resulting values 1 borehole with equilibrium water level below top of screen of K will be In actuality, however, source or perforated section,with outflow of water into vadose zone (horizontal arrows) boundaries of ground water flowing into the well in response to lowering the water level are hard to define because of elastic deformation of aquifer material and confining and interbedded fine- textured layers, and because of leakage through � case, the outfloK from the well due to the falling semiconfining layers water level occurs only through the screened or open section of the well, and the flog system in EFFECT OF WELL DIAMETER the aquifer is a true reverse of the flow system for the rising water level after a slug of rater has been Theoretically, the BouHer and Rice slug test removed (ignoring, of course, eventual risco and applies to any diameter of the borehole Practically, drawdowns of the ground-water tat3le in,mc dt ilcl% the hole dimensions should be selected so that the around the borehole if the aquilt geometry parameters arc covered by Figure 2 The Thus, equations (3), (4), and (5) } , k larger rN and L, (Figure 1), the larger the portion to the addition of a slug of rJtc „t of the aquifer on which K is determined For the subsequent rate of fall g,l -t ii,( layered aquifers, smaller values of L, may some- borehole for calculation o1 1, iyu.rt t:ound times be preferable because they give more resolu- the hole tion and more information about the vertical dis- If the equilibrium watci I(%Ll in the borehole tribution of K when the slug test is carried out at , is below the top of the scrccn or open section different depths Very small hole diameters (for (Figure 6), and water is added (hatched section in example 2 in or S cm) should still glee accurate Figure 6), the subsequent flow of eater into the values for K, but the i aloes apply to only a small aquifer due to the falling water level not only takes region around the cell and, hence, are more scnsi- place through the screen or perforations below the tive to spatial variability Also, inaccuracies in the original water table, but also through the vadose estimates of the thickness of gravel emclopes and zone above the original water table (arrows in developed zones have a greater effect on the calcu- Figure 6) This increases the rate of fall of the lated values of K where rc is small than where rc is water level in the borehole beyond that caused by large inflow into the aquifer and leads to an merestima- tion of K The greater the ratio of y/L(Figure 6) in PROCESSING OF y VERSUS t this case,the more the slug test will overestimate K MEASUREMENTS if the measurement is based on adding water to the To calculate 1/t 1n(yo/yt) for the appropriate hole and measuring the subsequent rate of fall of straight line portion of curves as in Figure 3 or 4, the water level two values of y on the straight line and their Ino /j )