Hydraulic Properties of Drainfield Trench Biomats formed in Georgia Soils. S.D. Finch, L.T. West, D.E. Radcliffe, and E.V.

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1 Hydraulic Properties of Drainfield Trench Biomats formed in Georgia Soils S.D. Finch, L.T. West, D.E. Radcliffe, and E.V. Hufstetler* Abstract Understanding hydraulic properties of biomats formed at the drainfield trench-soil interface is critical for understanding long-term wastewater acceptance rates for drainfield trenches. One of the effects of biomat formation is reduction in rate of wastewater movement from the dispersal trench into the soil. Therefore, the objectives of this study were to evaluate the impact of biomats on wastewater infiltration rates through drainfield trench bottoms and sidewalls, evaluate the thickness and porosity of the biomats, and use computer simulation to model two-dimensional wastewater flow from a conventional gravel system. To accomplish this objective, saturated hydraulic conductivity (K s ) measurements were taken on undisturbed cylindrical (9 cm diameter by 9 cm length) soil cores collected from the bottom and sidewall of dispersal trenches of mature onsite systems and from the adjacent un-impacted soil. Polished blocks and thin sections prepared from undisturbed samples were used to evaluate biomat thickness and porosity. Natural soil K s was less than 1.2 cm d -1 at five of six sites evaluated. Because of the low K of the soils and high variability in K measurements, reduction in K due to presence of a biomat was insignificant at sites these sites. At the site with the highest natural soil K s, the biomat significantly reduced water infiltration rate. Porosity evaluations of thick dark stained layers at the trench-soil interface suggest that only the upper part of this layer has appreciable pore infilling, and the dark coloration of the subjacent material is due to grain coatings instead of pore infilling. Simulated wastewater flows from the drainfield trench suggest that the trench bottom and sidewall contribute equally to wastewater infiltration into the soil. Introduction Onsite wastewater management systems (OWMS) are a cost effective, environmentally benign method to manage household wastewater if properly installed on suitable soils and properly maintained. In 1999, the U.S. Census Bureau estimated that 23 percent of the 115 million homes in the United States use OWMS to manage household wastewater including about 40% of homes in Georgia (USEPA, 2002). One feature that affects hydraulic performance of OWMS is development of a biomat or clogging mat due to accumulation of organic matter, suspended solids, microorganisms, and fine particles that plug pores at the soil dispersal trench interface. Biomat development clogs pores and reduces the long-term rate of wastewater infiltration (Laak, 1970; Otis, 1985, Siegrist, 1987) and eventually, may result in hydraulic failure of the system (Beach and McCray, 2003). The longevity of an OWMS is closely proportional to the rate of biomat formation and reduction of the long-term acceptance rate (Beach and McCray, 2003; White and West, 2003). *S.D. Finch, Graduate Research Assistant, L.T. West, Professor, D.E. Radcliffe, Professor, E.V. Hufstetler, Research Technician, Department of Crop and Soil Sciences 3111 Miller Plant Sciences Athens, GA

2 There is a lack of information on the impact of biomat formation on long term wastewater infiltration rates for soils and conditions in Georgia. Therefore, this study was initiated to evaluate changes in saturated hydraulic conductivity (K s ) of the biomat affected soil at the trench-soil interface for OWMS in Georgia as compared to K s of natural soils. The hypothesis was that biomat formation reduces wastewater infiltration rates of OWMS. The objectives of this study were to evaluate the impact of biomats on infiltration rates from OWMS, evaluate biomat thickness and porosity, and simulate wastewater flow from the drainfield trench into the soil using a 2-D computer model. Materials and Methods Field Methods Six sites were selected in the Georgia Piedmont and Coastal Plain with OWMS that ranged in age from 5 to 40 years. Triplicate cylindrical core samples (9 cm diameter by 9 cm length) were taken from the bottom and sidewall of onsite system drain field trenches at two or three locations in the drainfield. The cores were collected in a manner that included gravel (for systems with aggregate), biomat, and associated soil. Similar core samples with both vertical and horizontal orientation were collected from natural soils at depths corresponding to the drain field samples. Bulk samples of each horizon were also collected and undisturbed clods were taken for polished block and thin section preparation. Owners of each OWMS site provided the details of drain field design and age, while approximate loading rates were determined from the number of household occupants. Laboratory Methods Saturated hydraulic conductivity was measured on the core samples using the constant head method with a 0.1 M CaCl 2 solution to minimize clay dispersion (Klute and Dirksen, 1986). For cores from the drainfield trench, the measurement is an effective saturated hydraulic conductivity (K eff ) because the samples evaluated included both soil-biomat and natural soil layers (White and West, 2003). After K measurement, undisturbed samples were taken from the clods, dried by acetone replacement, and impregnated with polyester or epoxy resin containing a fluorescent additive. Polished blocks and thin sections were prepared by standard techniques (Murphy, 1986). Polished blocks were photographed under ultraviolet light to visually evaluate porosity differences between biomat affected and natural soil. Computer Simulation A simulation model, Hydrus-2D, was used to model two dimensional water flows in a gravel drainfield (Rassam et al., 2003). The drainfield configuration used for model simulations was a 90 cm wide and 30 cm deep gravel trench (only one-half of the drainfield was used for the simulations since it is horizontally symmetrical). Properties of the soil used for the simulations, sandy clay texture and K s of 3 cm d -1, were similar to those of horizons from which drainfield trench bottom and sidewall samples were 45 cm Soil Soil Biomat Figure 1. Half drainfield used for model simulations.

3 collected. Biomats at the trench-soil interface were 3 cm thick and had K s of 0.5 and 0.6 cm d -1 for trench bottom and sidewall, respectively. The rates are within the range typically reported for biomats on trench bottoms and sidewalls (Keys et al., 1998). Wastewater loading rate was 2 cm d -1 applied in three equal doses during the day at 8 am, 2pm, and 8pm. The simulation output that was evaluated was total flow (Q) per day through the trench bottom and sidewall per cm trench length (units of cm 3 cm -1 d -1 = cm 2 d -1 ). Results and Discussion Biomat thickness evaluated by visual macroscopic evaluation ranged from <1 to more than 30 mm (Figs. 2, 3, and 4). Variability in thickness was due to differences in system age, wastewater loading rate, system design, and method of wastewater distribution for the systems sampled. Biomat thickness was not uniform across the drainfields because of installation imperfections that resulted in uneven wastewater loading. Serial wastewater distribution within the drainfield also resulted in considerable differences in biomat thickness and hydraulic characteristics of the biomats sampled. Figure 2. Biomat sample from Site A with thickness of about 30 mm. Limited quantitative evaluations of biomat thickness collected to date suggest that the dark color typically used to estimate biomat thickness at a macro scale may overestimate thickness. In samples with a thick dark-colored layer, the upper part of the darkened layer had low porosity as would be expected from pore infilling by organic materials (Fig. 3). The lower part of the darkened layer, however, was more porous suggesting the dark color in this layer was from staining of grain surfaces instead of pore infilling (Fig. 3). The fabric and grain-size in the dark colored porous zone was different from that of the subjacent soil and was similar to the typical fabric of a sandy textured soil suggesting that sand may have been placed in the trench bottom during drainfield construction (Fig. 3). Homeowners with knowledge of drainfield installation, however, indicated that no sand had been added to the trench bottoms. Thus, it is hypothesized high organic loading and constant saturated or nearsaturated and anoxic conditions may have induced chemical reduction and oxidation of Fe and Mn at the soil-trench interface and protons generated in these redox reactions destroyed clay in this zone leaving the more resistant sand. The K s of natural vertical soils ranged from 0.02 to 3.2 cm d -1 and varied widely within and among sites (Fig. 5). The drainfield trenches sampled were typically installed in soil horizons with sandy clay loam or clay loam textures with weak structure. At a few of the sites, the horizon containing the drainfield also had weakly developed platy structure which would also restrict water movement through the horizon. In general, the permeability of soil horizons in which the drainfields was marginal for installation of drainfields, especially for serial distribution and the associated high hydraulic and organic loading.

4 Mean K eff of trench bottom samples was less than 1.8 cm d -1 at all sites (Fig. 5). The difference in K of the trench bottom samples and the natural soil was statistically significant only at Site A which had the highest natural soil mean K s of the sites evaluated, 3.2 cm d -1. Mean K eff of trench bottom samples tended to be higher that the K s of the natural soil at Sites C and F although these differences were not significant. Lack of significant reduction in K because of biomat development is attributed to variability in biomat development and to natural variability in K s of the soils. Because the K s of the natural soils was low, the difference in K between the natural and biomat impacted soil was small relative to variability in K among the samples. For the one soil with relatively high natural K s, the presence of a biomat significantly reduced water movement rates (Fig. 5). Figure 3. Polished blocks from Site C trench bottom impregnated with fluorescent resin. Left plain light; Right - reflected UV light. A region of biomat with low porosity; B region of biomat (or staining) with abundance of fine pores; C natural soils with linear structural pores. Scale in cm. Figure 4. Thin-section micrographs of biomat samples. Left-Pore clogging (black) from biomat. Plane-polarized light. Bar length = 0.5 mm. Right- Biomat on soil surface. Plane-polarized light. Bar length = 1 mm

5 K S (cm d -1 ) K S (cm d -1 ) Natural Vertical Trench Bottom Site A Site B Site C Site D Site E Site F Figure 5. Geometric mean of the saturated hydraulic conductivity of the trench bottom and natural vertical soil. Error bars represent the standard error of the mean Natural Horizontal Trench Sidewall Site B Site C Site D Site E Site F Figure 6. Geometric mean of the saturated hydraulic conductivity of the trench sidewall and natural horizontal soil. Error bars represent the standard error of the mean. The K s of horizontally-oriented natural soils ranged from 0.04 to 1.08 cm d -1 which was similar to K s of vertically-oriented natural soil samples (Fig. 5 and 6). The drainfield at Site A was a chamber system and meaningful biomat samples could not be collected at this site. The K eff for sidewall samples with a biomat was less than 1.23 cm d -1 at all sites except Site F. This site had an exceptionally high mean K eff that was significantly greater than the K s of the natural soil. The high K eff for the trench sidewall samples from Site F may be artifact related to sample collection. The first drainfield section of the serial distribution system was completely filled with wastewater at the time

6 of sampling. The saturated conditions at the trench-soil interface (the soil was dry 20 cm below and outside of the drainfield) resulted in soils with low strength, and soil disruption during sample collection may have occurred. Biomat impacted and natural soil samples had similar K at the other four sites (Fig. 6), and the K eff of the trench sidewall samples was similar to that of samples collected from the trench bottom. The sidewall samples were collected from the lower 10 cm of the trench sidewall where maximum biomat development would be expected, however (Keys et al., 1998). At many of the sites sampled, organic staining of the soils was noted as much as 30 cm laterally away from the drainfield trench, especially along foliations common in BC horizons of the soils developed in saprolite derived from metamorphic rocks. To better understand the relative contribution of trench bottom and sidewall to wastewater infiltration from the trench, a series of computer simulations were made. These simulations indicated that for a soil with properties similar to soils evaluated, about ½ of the total wastewater infiltration would be through the trench bottom and ½ through the trench sidewall (Fig. 7). Furthermore, under conditions of the simulations, about 80% of the wastewater infiltration through the sidewall was occurring in the zone without a sidewall biomat (upper 15 cm of the sidewall) (Fig. 8 and 9) which has been referred to as the sidewall lip (Keys et al., 1998). Beach and McCray (2003) reported similar results from modeling two-dimensional flow from drainfield trenches in a sand-textured soil. These authors found that the degree of pore clogging contributed to the relative amounts of trench bottom versus sidewall flow. The average flow rate from for a low clogged (4 cm) trench bottom was 29 cm d -1, while the flow rate from a high clogged (20 cm) system was 25 cm d -1. As flow rates, decreased due to pore clogging, there was a relative increase in the amount of flow through the sidewall. Conclusions Because of the low K s of the natural soils and variability in measured K among reps, the impact of biomat development on wastewater infiltration rates was insignificant at most of the sites evaluated in this study. The soil horizons in which the drainfield trenches were installed had relatively high clay contents (30 to 40%), were weakly structured, and often had platy structure. Because of these soil characteristics, K s of the horizons were low and only marginally hydraulically suited for onsite system drainfields. In these hydraulically-limited horizons, reduction in water movement rates due to biomat development would be expected to minor. At the site with the highest natural soil K s, the presence of a biomat significantly reduced the K eff of trench bottom samples by about 60%. Visual estimates of biomat thickness at a macro scale indicated biomat thickness to be up to 30 mm. Optical evaluations of porosity, however, suggested that only the upper part of the thicker biomats had appreciable pore infilling with the lower part of the darkened upper layer have relatively high porosity. Model simulations for a soil similar to those evaluated suggested that the trench bottom and sidewall had about equal amounts of wastewater infiltration with a biomat on the lower half of the trench sidewall. Most of the trench sidewall wastewater infiltration was into the upper portion of the sidewall without a biomat. These simulations suggest total wastewater infiltration from the drainfield trench will decrease over time as the sidewall biomat develops more completely, especially in systems or parts of systems that may be hydraulically overloaded.

7 Flow (cm d -1 ) Flow (cm d -1 ) Gravel Bottom Gravel Sidewall Time (d) Figure 7. Simulated steady state trench bottom and sidewall flow for gravel drainfield system for dispersal field dosed three times per day Sidewall w/ biomat Sidewall w/out biomat Time (d) Figure 8. Simulated steady state sidewall (with and without biomat) flows for gravel drainfield system for dispersal field dosed three times per day.

8 Distribution Pipe Natural soil w/out biomat Biomat Figure 9. Sidewall flow over the lip of the sidewall with biomat (Time 7.62 days). Arrows are flow vectors. Note that most simulated sidewall flow is in the upper half of the sidewall without a biomat. Literature Cited Biomat Beach, D.N.H., and J.E. McCray Numerical modeling of unsaturated flow in wastewater soil absorption systems. Ground Water Monit. Remediation 23: Hillel, D Introduction to environmental soil physics. Elsevier Science, USA. Keys, J.R., E.J. Tyler, and J.C. Converse Predicting Life for Wastewater Absorption Systems. IN Proceedings of the Eighth National Symposium of Individual and Small Community Sewage Systems. ASAE, Orlando FL. Klute, A. and C. Dirksen Hydraulic conductivity and diffusivity: Laboratory methods. p In A. Klute (ed.) Methods of soil analysis. Part 1. 2nd ed. Agron. Monogr. 9 ASA and SSSA, Madison, WI. Laak, R Influence of domestic wastewater pretreatment on soil clogging. J. Water Pollut. Control Fed. Part I. 42: Murphy, C.P Thin section preparation of soils and sediments. A B Academic Publishers. Berkhamsted, Great Britain. Otis, R.J Soil clogging: Mechanisms and control. p IN On-site wastewater treatment. Proceedings of the fourth national symposium on individual and small community sewage treatment. Am. Soc. Agric. Eng., St. Joseph, MI.

9 Rassam, D., J. Šimůnek, and M.Th. van Genuchten Modeling variably saturated flow with hyrdrus-2d. ND Consult. Brisbane, Australia. Siegrist, R.L Soil clogging during subsurfacce wastewater infiltration as affected by effluent composition and loading rate. J. Environ. Qual. 16: USEPA On-site wastewater treatment systems manual. EPA/625/R-00/008 Office of Water. Office of Research and Development. (Available on-line (Verified 13 Aug ) White, K.D. and L.T. West In-ground dispersal of wastewater effluent: The science of getting water into the ground. Small Flows Quarterly. 4 vol. no. 2