Amending Soils for Enhanced Infiltration of Stormwater

Abstract

Rainfall events on urban compacted soils increase the volume and rate of stormwater runoff. A research study was conducted recently in North Carolina to investigate soil amendments to enhance infiltration into compacted soils. Sites were located in the coastal plain, piedmont, and mountain regions and the sites had sand, sandy clay, and sandy clay loam textured subsoils, respectively. The soil profiles were exhumed into the subsoils and compacted to simulate urban disturbed conditions. Physical treatments/amendments included compacted (control), shallow tillage (15 cm), and deep tillage (30 cm). Fertility amendments included agricultural lime and fertilizer according to soil test results. Triplicate plots were randomized on each of the sites. Fescue grass was seeded, mulched, and covered with jute matting. Steady state infiltration rate, bulk density, cone penetrometer, grass shoot biomass, and grass root biomass measurements were taken over the study period. Runoff from natural rainfall events was measured for twelve storm events at the two piedmont sites. Tillage greatly increased the infiltration rates and the effect remained after three years. There was evidence of some decline in infiltration rates at the mountain site, but none at the other sites, even though the bulk densities tended to increase over time at all sites. Doubling recommended lime rates, or adding compost, or water absorbing polyacrylamide usually had no effect on infiltration rates. The initial infiltration rates for compacted soils were usually < 1 cm hr -1 but the rates improved over several years to up to 10 cm hr -1 . This was somewhat surprising, but the trend was evident at all sites. The tilled soils had infiltration rates of 20 to 35 cm hr -1 at the end of at least two years. Because this far exceeds expected rainfall of 3 to 6 cm hr -1 for 2 to 10 year recurrence storms, the results suggest that treated areas may be able to accept significant amounts of runoff from impervious areas.