Groundwater Monitoring at Earthen Manure-Storage Structures in Iowa
Robert D. Libra, Deborah J. Quade, and Lynette S. Seigley
Groundwater has been monitored monthly since 1993 at three earthen
manure storage structures in Iowa. Seepage has been detected in all downgradient wells at
the Des Moines Lobe site in north-central Iowa (earthen basin) and at the Iowan Surface
site in east-central Iowa (2-cell earthen lagoon). Indications of seepage are similar and
include: the decline or loss of nitrate-N and sulfate, and an increase in concentrations
of chloride and total organic carbon. Chloride concentrations in the berm well at the
north-central Iowa site are 40% of those measured in the liquid waste; at the east-central
Iowa site, concentrations at the closest well are 80% of those in the waste. Fecal
coliform bacteria have been sporadically detected in the closest well at the north-central
Iowa site. Concentrations of nutrients, such as ammonia-N, initially did not increase,
indicating these species were being retained by cation (positively charged ion) exchange.
However, ammonia-N concentrations have recently shown increases, suggesting the exchange
capacity is being depleted. Organic-N concentrations have increased, and are typically
higher than ammonia-N concentrations. Exchange reactions that limit ammonia-N transport
result in a build-up of ammonia-N beneath an earthen manure-storage structure. At the
allowable seepage rate of 1/16th inch/day, a basin like the one used at the
north-central Iowa site (4,500-head hog finishing operation) would transport 5,300 pounds
of ammonia-N to the glacial materials beneath the ½-acre basin annually.
During the 1990s, large-scale hog confinement operations began appearing in Iowa. A majority of the largest operations utilize earthen structures --either basins or anaerobic lagoons-- to store the produced manure until it can be applied to crops. Concerns were raised regarding the potential for groundwater contamination from these earthen manure-storage structures. In 1993 the Iowa Department of Natural Resources began monitoring shallow groundwater quality at three structures used for concentrated hog production. These structures are similar to those currently being constructed elsewhere in Iowa. They are located in areas with differing surficial geologic materials: the Des Moines Lobe in north-central Iowa, the Iowan Surface in east central Iowa (Kirkwood Community College), and the thick loess region of western Iowa (Figure 1). Three to seven shallow (<25 ft.) monitoring wells were installed around each structure. Water levels were measured and water samples were collected monthly for nitrate-N, ammonia-N, fecal coliform bacteria, and chloride analysis. Samples were analyzed quarterly for total organic carbon, sulfate, phosphate, and other parameters. Since the fall of 1996, monitoring has occurred only on a quarterly basis. Seepage has been detected at the north-central and east-central Iowa sites (discussed below). Monitoring was discontinued the summer of 1998 at the site located in the thick loess region of western Iowa, as no seepage was detected at this site.
North-Central Iowa Site
Surficial deposits at the north-central Iowa site consist of uniform, dense, loam-textured subglacial till, overlain by 12-15 feet of supraglacial till with low bulk density values and highly variable textures, which vary from loam, to sandy loam, to sand (Quade et al., 1996; 1998) (Figure 2). An earthen basin, constructed from supraglacial till, is used to store wastes from a 4,500-head finishing operation. The basin began receiving wastes in January of 1994. Seven monitoring wells were installed around the basin (Figure 3). Grab samples of the waste liquid were collected six times (Table 1). Chloride concentrations were relatively consistent, at 490-1,100 mg/L. Ammonia-N concentrations were more variable, ranging from 1,600-4,400 mg/L. The reason for this large difference is not known. Nitrate concentrations were negligible in the waste liquid. Phosphate-P concentrations were determined once, and were 200 mg/L.
Table 1. Concentrations in samples of liquid waste collected from the north-central Iowa earthen basin and east-central Iowa lagoon.
Varying indications of seepage from the basin have been detected at all downgradient wells. Figure 4 shows pertinent monitoring results from well Kan-5, which samples the groundwater immediately after it has passed beneath the basin. Indications of seepage include increases in the concentrations of chloride and total organic carbon (TOC), and a sharp decline in nitrate-N and sulfate concentrations. Chloride concentrations began increasing about three months after the basin began receiving wastes, reaching 400 mg/L after three years. A mass balance based on the highest chloride concentrations indicates the water sampled at well Kan-5 was a mixture of about 55% groundwater and 45% basin seepage. Chloride concentrations then declined to about 325 mg/L before increasing again. The continuing high chloride concentrations indicate the manure has not "sealed" the basin (for information on sealing see Barrington and Jutras, 1983; Miller et al., 1985; Rowsell et al., 1985).
TOC concentrations at well Kan-5 increased from less than 10 mg/L to over 200 mg/L during the summer of 1994, and have varied from 30 mg/L to over 1,000 mg/L since then. There appears to be a seasonal trend to the data, with higher concentrations occurring in the summer. The decrease in nitrate-N and sulfate concentrations is a result of denitrification and sulfate reduction, respectively, generated by the anaerobic, organic-carbon rich nature of seepage from the basin. Fecal coliform bacteria were present in 21 of 34 monthly samples. Concentrations between 10 and 100 colonies/100 ml are most common, and are higher during summer, which suggests the bacteria are surviving longer, and are more likely to be transported to well Kan-5, under warmer conditions.
While the chloride data from well Kan-5 clearly show the basin is seeping, the major nutrients ammonia-N and phosphate (and presumably potassium) have not shown a similar response. Total phosphate-P concentrations have remained below the 0.5 mg/L detection limit. Ammonia-N concentrations have ranged from near zero to about 5 mg/L, but have often been below 1 mg/L. If ammonia-N and phosphate-P were being transported from the basin as effectively as the conservative chloride ion, water from well Kan-5 would likely contain over 1,000 mg/L ammonia-N and 100 mg/L phosphate-P. This indicates these nutrients are largely being retained, via cation-exchange processes, on the clayey materials below the basin. A considerable build-up of nutrients may be occurring below the basin, depending upon the actual volume of seepage (Figure 5). For perspective, the maximum allowable seepage rate for earthen waste structures in Iowa is 1/16th inch/day. If the north-central Iowa basin were seeping at this limit, roughly 5,300 pounds of ammonia-N would be retained on the glacial materials beneath the ½-acre basin each year. While ammonia-N is largely retained below the basin, concentrations appear to be increasing over time. This suggests the adsorptive or cation-exchange capacity of the glacial materials beneath the structure is being exceeded, and that further increases in ammonia-N transport to well Kan-5 will occur. Organic-nitrogen concentrations have occasionally been measured, and routinely exceed ammonia-N concentrations.
The farthest downgradient well at the north-central Iowa site is located 150 feet away (well Kan-7; Figure 6). Water-quality trends are similar to those found at well Kan-5, but seepage-related changes are less significant. A mass-balance based on chloride suggests the groundwater passing 150 feet from the basin contains 15% seepage.
East-Central Iowa Site
The geology of the east-central Iowa site is characterized by fractured Pre-Illinois glacial till, mantled by colluvium (slopewash materials) and a thin deposit of windblown silt (loess). Wastes from a 130-sow farrowing and 680-hog finishing operation are stored in a two-cell lagoon, constructed primarily in uniform loam textured, fractured till. Five monitoring wells were installed at this site (Figure 7). Samples of waste were collected five times from the north lagoon cell, with higher concentrations observed in the later sampling (Table 1). Ammonia-N concentrations varied from 170-830 mg/L, and chloride ranged from 160-670 mg/L. Note that concentrations are considerably lower than those measured for the liquid waste at the north-central Iowa earthen basin. This shows the more dilute nature of the wastes in a lagoon, relative to a basin (Table 1). It is unclear why the fecal coliform bacteria levels were higher in the waste from the lagoon relative to the basin.
Seepage has been detected at all downgradient wells at the east-central Iowa site. Figure 8 shows the results of the monitoring at well K-4, which is drilled through the downgradient berm of the north lagoon cell. Chloride concentrations followed an increasing trend across the period, from around 50 to over 350 mg/L, indicating seepage from the basin to the water table. A mass-balance based on chloride concentrations suggests that the groundwater flowing out from the lagoon was comprised of about 80% seepage during the last half of the monitoring period. As chloride concentrations have not declined, the lagoon has not "sealed." Nitrate-N concentrations were below 0.5 mg/L for all samples other than the initial collection in October 1993, when 1 mg/L nitrate-N was present. Sulfate concentrations declined through the period, from about 25 to 5 mg/L. The relatively low initial concentrations, and their continued decline, is suggestive of denitrification and sulfate reduction, as was noted for the downgradient wells at the north-central Iowa site.
TOC concentrations were less than 10 mg/L for the first samples that were collected in 1994. Samples collected during the remainder of the period ranged from 50 to 400 mg/L. The increase likely reflects the input of TOC-rich seepage to the water table. Fecal coliform bacteria were detected in only four monthly samples, at concentrations of 2 to 20 mg/L. Fecal coliforms do not appear to be routinely transported from the north lagoon cell. Ammonia-N concentrations remained below 1 mg/L and appeared to be following a general declining trend through the spring of 1996. Since the summer of 1996, however, ammonia-N concentrations have increased rather sharply, exceeding 5 mg/L in the fall of 1997. The liquid waste in the north cell contains about 300 mg/L ammonia-N and 70 mg/L phosphate-P, and chloride analyses indicate that the groundwater at K-4 is roughly 80% seepage. This suggests that if ammonia-N and phosphate-P were being transported from the basin and to well K-4 as readily as chloride, samples from well K-4 would have contained around 200 mg/L ammonia-N and 50 mg/L phosphate-P during the latter half of the monitoring period. Ammonia-N and phosphate-P appeared to be effectively adsorbed and retained by the sediments beneath the lagoon cell. However, ammonia-N concentrations are beginning to rise, suggesting the exchange capacity of the sediments below the basin has been utilized. Organic-nitrogen concentrations have occasionally been measured, and routinely exceed ammonia-N concentrations. Ammonia- plus organic-N in September 1997 exceeded 20 mg/L.
The farthest downgradient well at the east-central Iowa site, well K-2, is located about 150 feet away. Water-quality trends are similar to those found at well K-4, but seepage-related changes are less significant (Figure 9). A mass-balance based on chloride suggests the groundwater passing 150 feet from the basin contains 40% seepage.
Seepage to the water table is occurring at two of the three monitored structures. Indications of this seepage are generally similar at all affected monitoring wells, and include rising concentrations of chloride and organic carbon, and declining concentrations of sulfate and nitrate. Organic-N concentrations appear to be increasing. Concentrations of nutrients such as ammonia-N and phosphate-P have not significantly changed from background conditions until the latter part of the monitoring period. Adsorption of these nutrients has largely removed them from the seepage. However, the nutrients could be remobilized under different geochemical conditions, and therefore remain a potential source for future contamination. Movement of fecal coliforms from the structures does not seem significant. The structures where seepage is readily identified have not sealed during four years of operation.
A number of previous studies have attempted to evaluate the effects of
earthen manure-storage structures on groundwater quality. Parker and others (1994)
provided a comprehensive review of literature on the topic. Most of these studies were
conducted in areas with relatively permeable, coarse-grained surficial deposits. Earthen
manure-storage structures built in such environments have been shown to cause increased
concentrations of chloride and nutrients, such as ammonia-N (which may convert to
nitrate-N), phosphorus, potassium, and other waste constituents in nearby shallow
groundwater (Ritter et al., 1984: Sewell, 1978; Westerman et al., 1993). The permeable
deposits described in these studies allow for greater rates of seepage and groundwater
flow than the more clayey glacial deposits beneath the Iowa earthen manure-storage
structures, and additionally, have little cation exchange capacity. Therefore, ammonia-N
and other cations are not significantly retained by these coarse-grained materials.
The results discussed above are from only three of the over six hundred earthen manure-storage structures that have been constructed in Iowa in the last seven years. They are from structures that are relatively new, having been in use four years or less. They are also from "moderately-sized" operations that have been constructed on predominantly fine-grained, clayey materials. It is unknown whether the results of this investigation are typical of other earthen manure-storage structures. In particular, monitoring of earthen structures built on -- and of -- coarser materials, structures that have been in service longer, or significantly larger structures, may yield different results.
Barrington, S.F., and Jutras, P.J., 1983, Soil sealing by manure in various soil types. A.S.E.A. Paper #83-4571, St. Joseph MI, 17 p.
Miller, M.H., Robinson, J.B., and Gillham, R.W., 1985, Self-sealing of earthen liquid manure storage ponds: I. A case study: Journal of Environmental Quality, v.14, #4, p. 533-538.
Parker, D.B., Schulte, D.D., Eisenhauer, D.E., and Nienber, J.A., 1994, Seepage from animal waste lagoons and storage ponds - regulatory and research review. Proceedings, Great Plains animal waste conference on confined animal production and water quality: Balancing animal production and the environment. Great Plains Agricultural Council publication #151, p. 87-98.
Quade, D.J., Libra, R.D., and Seigley, L.S., 1996, Groundwater monitoring at an earthen manure-storage structure, in Hogs, Bogs, and Logs: Quaternary Deposits and Environmental Geology of the Des Moines Lobe, E.A. Bettis, III, D.J Quade, and T.J Kemmis (eds.), Iowa Department of Natural Resources-Geological Survey Bureau, Guidebook Series No. 18, p. 141-153.
Quade, D.J., Libra, R.D., and Seigley, L.S., 1998, Stop 5: Groundwater monitoring trends at an earthen manure-storage structure, in Fossil Shells, Glacial Swells, Piggy Smells, and Drainage Wells, Geological Society of Iowa Guidebook 65, p. 51-61.
Ritter, W.F., Walpole, E.W., and Eastburn, R.P., 1984, Effect of an anaerobic swine lagoon on groundwater quality in Sussex County, Delaware: Agricultural Wastes, Elsevier Applied Science Publishers, England, p. 267-284.
Rowsell, J.G., Miller, M.H., and Groenevelt, P.H., 1985, Self-sealing of earthen liquid manure storage ponds: II. Rate and mechanism of sealing: Journal of Environmental Quality, v. 14, #4, p. 539-543.
Sewell, J.L., 1978, Dairy lagoon effects on groundwater quality: Transactions of the A.S.A.E., v. 21, p. 948-952.
Westerman, P.W., Huffman, R.L., and Seng, J.S., 1993, Swine-lagoon seepage in sandy soil: A.S.A.E. paper #93-4527, 32 p.
Guidebook for field trip to the Kirkwood Community College Animal Confinement Facility, conducted as part of the Sixth National Nonpoint-Source Monitoring Workshop, September 21, 1998, Cedar Rapids, IA, p. 5-14.