Evaluation of soils for use as liner materials: A soil chemistry approach

Movement of NH4+ below animal waste lagoons is generally a function of the whole-lagoon seepage rate, soil mineralogy, cations in the lagoon liquor, and selectivity for NH4+ on the soil-exchange sites. Binary exchange reactions (Ca2+-K+, Ca2+-NH4+, and K+-NH4+) were conducted on two soils from the Great Plains and with combinations of these soils with bentonite or zeolite added. Binary exchanges were used to predict ternary exchanges (Ca2+-K+-NH4+) following the Rothmund-Kornfeld approach and Gaines-Thomas convention. Potassium and NH4+ were preferred over Ca2+ and K+ was preferred over NH4+ in all soils and soils with amendments. Generally, the addition of bentonite did not change cation selectivity over the native soils, whereas the addition of zeolite did. The Rothmund-Kornfeld approach worked well for predicting equivalent fractions of cations on the exchanger phase when only ternary-solution phase compositions were known. Actual swine- and cattle-lagoon solution compositions and the Rothmund-Kornfeld approach were used to project that native soils are predicted to retain 53 and 23%, respectively, of the downward-moving NH4+ on their exchange sites. Additions of bentonite or zeolite to soils under swine lagoons may only slightly improve the equivalent fraction of NH4+ on the exchange sites. Although additions of bentonite or zeolite may not help increase the NH4+ selectivity of a liner material, increases in the overall cation exchange capacity (CEC) of a soil will ultimately decrease the amount of soil needed to adsorb downward-moving NH4+.