Saturated and unsaturated hydraulic conductivity of synthetic gel structures in coarse textured soil substrates

Abstract

Saturated and unsaturated hydraulic conductivity as a function of a thermodynamic potential (pressure) of soil water K(P) are investigated in the sandy samples with admixtures of strongly swelling polymer hydrogels (SSPHs) in a wide concentration range from 0.01 to 0.3 %SSPHs. The author’s original centrifugation method has been used along with a kinetic model of the water removal process, which enables to determine the function of water conductivity of gel compositions operating in an unsteady state. In saturation state and within the range of low absolute values of soil water pressure |P| = 0.3-20 kPa the SSPHs significantly (2-10 up to 50 times) reduced the hydraulic conductivity in proportion to the dose of a superabsorbent. A heterogeneous layered structure with a layer of pure gel or gel composition 0.1-0.3% SSHP reduces the saturated conductivity by 10-80 times.In the range | P | = 100-3020 kPa, the K(P) function, on the contrary, strongly increased up to 10–20 times under the action of SSPHs. The calculation of K(P) using the Mualem theory disagrees with our experimental estimation, especially in a range of low water content, where such a calculation strongly (up to 100-1000 times) underestimates the values of K(P).

The Impacts of Bio-Based and Synthetic Hydrogels on Soil Hydraulic Properties: A Review

“…The effect of hydrogels on saturated hydraulic conductivity ( Ks ) has been inconsistent though most studies on soil hydraulic conductivity have focused on Ks (as opposed to unsaturated hydraulic conductivity). Out of the 14 studies surveyed that investigated the effects of hydrogels on Ks , nine of the 14 studies indicated a decrease in Ks [ 10 , 32 , 45 , 57 , 64 , 65 , 74 , 83 , 87 ], however, three studies [ 28 , 66 , 88 ] reported an increase and three studies reported an initial decrease in Ks then a subsequent increase in Ks with time [ 60 , 88 , 89 ]. In two of the studies [ 60 , 89 ], the subsequent increase in Ks seem to increase over the original Ks .…”

“…Much of the evidence for decreases in Ks was found in studies that used synthetic hydrogels. A total of six studies [ 32 , 45 , 57 , 74 , 83 , 87 ] using various synthetic hydrogels definitively argue that hydrogels decrease Ks . Four of the six studies applied hydrogels to sandy soils underscoring the need for more studies using other soil textures.…”

“…The author explained that, as hydrogel concentration increased, the swelling of the hydrogel decreased the paths available for downward movement of water. Smagin et al [ 87 ] noticed that partially swollen hydrogels decreased Ks by up to 3.2 times compared to when dried hydrogel was applied, which gave a 1.4-fold reduction in Ks . They therefore advised that hydrogels be applied in swollen form to gain full benefits of reduction in Ks .…”

“…Like Ks , unsaturated hydraulic conductivity ( K ) is important for the movement of water in soil and more than Ks in field conditions [ 86 ], although fewer studies have investigated the impacts of hydrogels on K . A survey of literature found two studies since the year 2000 that measured K after applying hydrogel [ 71 , 87 ]. Both studies reported a decrease in K with the application of hydrogel.…”

“…Their results reveal a decrease in K of 85.5 to 94.1% on day 0, 75.1 to 82.9% on day 30 and 65 to 76.2% on day 50. Smagin et al [ 87 ] suggested that at high matric potentials i.e., >−10 to −15 kPa, K was reduced up to 2–3 times at hydrogel concentrations ranging from 0.01–0.05% (w/w) and a reduction of 10–50 times at 0.1–0.2% hydrogel concentration. However, at low matric potentials i.e., −20 to −700 kPa, K increased with an increase in hydrogel application rate.…”