eakage from a representative sample of 28 earthen manure storage structures and lagoons (selected from 459 built in Iowa between 1 January 1987 and 31 December 1994) was determined using a water–balance approach. Forty–three percent (43%) of tested structures had leakage rates significantly (p < 0.05) lower than the regulatory limit of 1.6 mm/d (1/16 in/d) specified by the State of Iowa at the time the basins were constructed. Leakage from 53% of the structures was too close to the regulatory limit to be categorized as being significantly above or below it. One structure (4%) exhibited leakage significantly greater than the regulatory limit. Regression analysis indicates a slight, but statistically significant, decline in leakage rate with increasing structure age. Structures constructed in glacial till showed significantly lower leakage rates than those constructed in sand and gravel, colluvium, or loess. Comparison of slurry pits and lagoons showed no significant difference in leakage rate.