{"id":1803,"date":"2026-02-14T04:27:45","date_gmt":"2026-02-14T04:27:45","guid":{"rendered":"https:\/\/books.gw-project.org\/an-introduction-to-hydraulic-testing-in-hydrogeology\/chapter\/ch078-xhtml\/"},"modified":"2026-08-01T11:00:33","modified_gmt":"2026-08-01T11:00:33","slug":"test-methods-used-to-estimate-aquitard-properties","status":"publish","type":"chapter","link":"https:\/\/books.gw-project.org\/an-introduction-to-hydraulic-testing-in-hydrogeology\/chapter\/test-methods-used-to-estimate-aquitard-properties\/","title":{"raw":"16.2 Test Methods Used to Estimate Aquitard Properties","rendered":"16.2 Test Methods Used to Estimate Aquitard Properties"},"content":{"raw":"
\r\n
\r\n
\r\n\r\nThis section focuses on the physical characterization of hydrogeologic properties of aquitards. In this book, we purposefully use the terms pumping test and hydraulic test instead of aquifer test. Conceptually, the pumping\u2011test techniques described in previous sections are applicable to both highly conductive and, in some settings, lower\u2011permeability units that may be considered aquitards. This section addresses some of the specific methods used to characterize the hydrogeologic properties of aquitards. Cherry and others (2006) and Bradbury and others (2006) also describe methods used to evaluate the integrity of aquitards as this condition relates to the potential for migration of contaminants from an impacted source through an aquitard.\r\n\r\n<\/div>\r\n
\r\n\r\nCherry and others (2006) suggest that characterization of aquitard hydraulic properties can be grouped into the application of external and internal methods. Internal methods include laboratory methods applied to samples and cores of the aquitard, performing hydraulic tests within a borehole penetrating the aquitard, and observing head responses to recharge or loading. External methods involve pumping a productive water\u2011bearing unit above or below an aquitard and then interpreting time\u2011drawdown data from observation wells in the principal water\u2011bearing unit and, in some cases, in the adjacent aquifer and\/or aquitard.\r\n\r\n<\/div>\r\n
\r\n

Internal Methods<\/h3>\r\n
\r\n\r\nDirect internal investigation of an aquitard includes collecting and testing sample cores as well as installing wells and conducting pumping and\/or slug tests. Indirect internal methods include monitoring the response of the near surface unconsolidated aquitard material to recharge events and monitoring the aquitard\u2019s response to external loading.\r\n\r\n<\/div>\r\n
\r\n\r\nLaboratory Methods\r\n\r\n<\/div>\r\n
\r\n\r\nResearchers have observed that hydraulic\u2011property values estimated from laboratory investigations of samples of uniform aquitards without significant secondary affects (developed macropores and\/or fracturing) may be equivalent to those derived from field\u2011scale methods (Cherry et al., 2006). However, laboratory\u2011scale samples of heterogeneous materials may poorly represent field\u2011scale properties, as the density and general interconnectedness of zones of different lithologies and secondary permeability features may be missing, overrepresented, or underrepresented at the laboratory scale. In some cases, laboratory samples are repacked and yield unrepresentative results. When any of these conditions occur, laboratory\u2011derived hydraulic conductivity values often underestimate field\u2011scale permeability (Haefner, 2000; Neuzil, 1986; van der Kamp, 2001).\r\n\r\n<\/div>\r\n
\r\n\r\n<\/span>Box 9<\/a> illustrates some of the laboratory methods such as the falling\u2011head permeameter, triaxial cell, and consolidometer that are used to derive hydraulic conductivities of low\u2011permeability material.\r\n\r\n<\/div>\r\n
\r\n\r\nField Methods\r\n\r\n<\/div>\r\n
\r\n\r\nThe following subsections describe internal field methods for characterizing hydrogeologic properties of aquitards.\r\n\r\n<\/div>\r\n
\r\n\r\nPumping tests\r\n\r\n<\/div>\r\n
\r\n\r\nConceptually, a pumping well and observation wells could be constructed in an aquitard. In most settings, the low yield potential and permeability of the unit would limit the pumping rate and the distance at which observation wells would respond to pumping. In addition, if more permeable units underlie or overlie the aquitard, they must be accounted for in the analysis. In some settings, pumping a single well at a low constant rate with measurements of drawdown and time may be most useful. This is discussed in Section 12. If a successful pumping test is conducted, drawdown responses in the aquitard would be analyzed using methods described in Sections 7 through 10.\r\n\r\n<\/div>\r\n
\r\n\r\nSlug Tests\r\n\r\n<\/div>\r\n
\r\n\r\nSlug tests, as described in Section 14, determine internal horizontal hydraulic conductivities of screened portions of aquitards. Hydraulic\u2011conductivity values reflect the nature of the material within a few meters of the test interval. The local nature of the test may not capture conditions where secondary permeability or material heterogeneity dominate the formation. Slug testing does not provide direct measurement of the vertical hydraulic conductivity. Estimates of vertical hydraulic conductivity are often assumed to be an order of magnitude smaller than horizontal values in stratified rock or sediments. However, in some settings horizontal to vertical hydraulic conductivity ratios can be 1000:1 (Anderson et al., 2015). In contrast, aquitard vertical hydraulic conductivity may be greater than horizontal values in laterite or saprolite materials where remnant structures such as weathered quartz veins may be present (Cherry et al., 2004).\r\n\r\n<\/div>\r\n
\r\n\r\nChanges in the near\u2011well hydraulic conductivity can occur when wells and piezometers are installed in unlithified aquitards (e.g., Cherry et al., 2006). Drilling can smear silt and clay along the borehole wall, reducing permeability. This skin effect artificially slows the rate of the head response causing hydraulic conductivity values to be underestimated. If the boring, well\u2011completion, and development processes enhance the permeability, values of hydraulic conductivity at the well will be overestimated. Perforated intervals should be developed prior to testing (e.g., Sterrett, 2007).\r\n\r\n<\/div>\r\n
\r\n\r\nSlug\u2011test responses in aquitards are overdamped as discussed in Section 12. In aquitards, head changes during a slug test occur slowly and can take hours to days or months to fully recover. In some low\u2011permeability settings, the static water level after the drilling and completion of a piezometer or monitoring well may be below the fully recovered head. Often, collection of water\u2011level data as recovery occurs can be used as a slug\u2011out test and analyzed accordingly as explained in Section 14. Often in low\u2011permeability materials, monitoring wells with small diameters (smaller volumes to be filled) are used to observe head change. Aquitard well design must accommodate slugs or pneumatic methods, and water\u2011level monitoring tools.\r\n\r\n<\/div>\r\n
\r\n\r\nConstant\u2011Drawdown Tests\r\n\r\n<\/div>\r\n
\r\n\r\nCharacterization of a low\u2011permeable unit can also be evaluated using a constant\u2011drawdown method. This method usually involves the head in the well to rise or fall and then at some point during the recovery water is continuously injected or extracted at varying flow rates to keep the head constant. Data collected are the well\u2011design factors and the inflow or outflow rate, Q<\/em>, needed to keep the unrecovered head (H<\/em>) constant (e.g., Mieussens & Ducasse, 1977; Tavenas et al., 1990; Neville & Markle, 2000) as illustrated in Figure 141<\/a>.\r\n\r\n<\/div>\r\n
\r\n\r\n\"\"\r\n\r\n<\/div>\r\n
\r\n
\r\n
\r\n\r\nFigure 141 \u2011\u00a0<\/strong>Slug\u2011out constant drawdown test set up for a monitoring well. The dashed blue line is the static water level. A slug of water is removed from the well (H0<\/sub><\/em>) and at some time during the recovery the unrecovered head, H<\/em>, in the well is stabilized by extracting volumes of water over time. The test time, t<\/em>, starts when the variable discharge begins. L<\/em> is the length of the perforated interval and d<\/em> is the diameter.<\/span>\r\n\r\n<\/div><\/blockquote>\r\n<\/div>\r\n
\r\n\r\nTavenas and others (1990) developed a method to analyze the data using plots of Q<\/em>(t<\/em>) versus 1\/t<\/em>0.5<\/sup>. This relationship should be linear after 5 to 10 minutes from the test initiation. Using an arithmetic plot, a line is fitted to the linear portion of the data and then extrapolated to 1\/t<\/em>0.5<\/sup> = 0. The steady\u2011state flow, Qinfinity<\/sub><\/em>, is determined by the intersection with the y axis. Using the steady\u2011state flow rate, Kh<\/sub><\/em> is computed as shown in Equation\u00a0(127)<\/a>.\r\n\r\n<\/div>\r\n<\/colgroup>\r\n\r\n\r\n
<\/th>\r\n[latex]K_{h} = \\ \\frac{Q_{infinity}}{F\\ H}[\/latex]<\/th>\r\n(127)<\/span><\/th>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n
\r\n\r\nwhere:\r\n\r\n<\/div>\r\n<\/colgroup>\r\n\r\n\r\n\r\n\r\n\r\n\r\n
Kh<\/sub><\/em><\/th>\r\n=<\/th>\r\nhorizontal hydraulic conductivity (LT\u20111<\/sup>)<\/th>\r\n<\/tr>\r\n<\/thead>\r\n
Qinfinitity<\/sub><\/em><\/td>\r\n=<\/td>\r\nSteady\u2011state flow rate from graph (L3<\/sup>T\u20111<\/sup>)<\/td>\r\n<\/tr>\r\n
F<\/em><\/td>\r\n=<\/td>\r\nshape factor for the monitoring well (function of radius, screen length and other factors) (L)<\/td>\r\n<\/tr>\r\n
H<\/em><\/td>\r\n=<\/td>\r\nunrecovered head as illustrated in Figure 141<\/a> (L)<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n
\r\n\r\nThe shape factor can be approximated as suggested by Hvorslev (1951) and shown in Equation\u00a0(128)<\/a>.\r\n\r\n<\/div>\r\n<\/colgroup>\r\n\r\n\r\n
<\/th>\r\n[latex]F\\ = \\ \\frac{2\\ \\pi\\ L}{\\ln\\left( \\frac{L}{d} + \\ \\left( 1 + \\left( \\frac{L}{d} \\right)^{2} \\right)^{.5} \\right)}[\/latex]<\/th>\r\n(128)<\/span><\/th>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n
\r\n\r\nwhere:\r\n\r\n<\/div>\r\n<\/colgroup>\r\n\r\n\r\n\r\n\r\n
L<\/em><\/th>\r\n=<\/th>\r\nperforated length (L)<\/th>\r\n<\/tr>\r\n<\/thead>\r\n
d<\/em><\/td>\r\n=<\/td>\r\ndiameter of the perforated interval (L)<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n
\r\n\r\nTavenas and others (1990) provide additional discussion of parameters. The Jacob\u2011Lohman (1952) solution as outlined by Lohman (1972) may also be applicable for analyzing constant\u2011head tests. This method assumes a constant head from a flowing well where the change in discharge over time is recorded.\r\n\r\n<\/div>\r\n
\r\n\r\nWhen aquitard material lies at or near the surface, permeability has been investigated using other methods referenced by Cherry and others (2006). These include monitoring and analyzing head changes within the aquitard induced by\r\n\r\n<\/div>\r\n