Reduced Disposal Area Performance Utilizing Secondary-Treated Effluent in Profile-Limiting Soils

Onsite wastewater systems dispose of primary treated effluent by utilizing the soil for final recycling and renovation of wastewater into the environment. Soil and site limitations have become a challenge to design a wastewater system and dispose of onsite wastewater using a conventional pipe and gravel design. Using secondary-treated effluent from an advanced treatment unit applied to a reduced disposal area offers an additional alternative when developing an onsite wastewater system. The objective of this study was to de-termine the feasibility of hydraulically loading limiting soils with second-ary-treated effluent in a reduced disposal area. A reduced disposal area was constructed at six existing residences within the same subdivision that had shallow redoximorphic features that precluded using a conventional pipe and gravel wastewater design. Each residence had an existing advanced treatment unit with a surface discharge of secondary-treated effluent. Flows were diverted from the surface discharge to the reduced disposal area. Wastewater flows were recorded at regular intervals, along with ponding depths in the disposal area and fluctuations in the seasonal water table over a 12-month period (March 2017 to March 2018). The disposal areas were hydraulically loaded at 2 to 3.8 times the rate recommended for secondary-treated effluent. Wastewater effluent was sampled throughout the study and resulted in a mean of <8.5 mg∙L −1 total suspended solids, <5.3 mg∙L −1 biochemical oxygen demand, and >6.3 mg∙L −1 dissolved oxygen, all of which met or exceeded the minimum water quality criteria for surface discharges of secondary-treated


Introduction
Managing household wastewater (i.e. effluent) by onsite disposal is critical to keeping rural areas and water sources free from disease and unsanitary living conditions. According to the 2017 Rural Profile of Arkansas, 42% of the Arkansas population is classified as rural [1]. Consequently, rural dwellings in Arkansas that are not connected to a public sewer system must utilize an onsite wastewater system that relies on the soil to renovate household wastewater before the effluent is returned to the hydrologic cycle.
In 1977, the Arkansas General Assembly passed the Sewage Disposal Act 402.
Act 402 defined the guidelines for handling domestic waste. Following Act 402, the Arkansas Department of Health adopted Rules and Regulations regarding onsite wastewater disposal [2]. The Rules and Regulations are referenced today and revised periodically with improved methods and technologies. Each day, wastewater from rural Arkansas homes and businesses is discharged into the soil where the effluent is renovated by filtering through the soil and recycled back into the environment using conventional onsite wastewater systems. The soil captures and clarifies the effluent from a wastewater system by removing nutrients, pathogens, and remaining suspended solids [3]. A soil disposal area is the most efficient and cost-effective method to dispose of wastewater. The daily discharge of wastewater into the soil over time, with little evidence of contaminants or unsanitary conditions, shows how efficient the soil can be in renovating wastewater. However, as Arkansans continue to develop more rural areas that require an onsite wastewater system, locating suitable soil to safely renovate wastewater has become a challenge due to limiting soils (i.e. shallow depth to bedrock, a shallow seasonal water table, or >35% clay textures) or a limited suitable disposal area available on the property.
An unsewered property being considered for development with limiting soils that is not suitable for a conventional wastewater system is allowed to utilize an advanced treatment unit (ATU) to manage the wastewater output. Advanced treatment units generate secondary-treated effluent, which allows for dispersal to a drip disposal area utilizing the soil for final renovation or an ATU with an overland-flow discharge. Drip disposal may be considered in limiting soils, if the soils meet the minimum suitability requirements defined in the Arkansas Drip Rules and Regulations. The drip disposal tubing must be installed at a depth with at least 23 cm separation from bedrock and may not be installed in any seasonal water table [4]. An advantage is that drip disposal can be utilized in limiting soils where a conventional wastewater system cannot. Disadvantages to utilizing drip disposal include the requirements of an additional license to design Currently, there are no data to provide guidance for using the soil to renovate secondary-treated effluent in Arkansas. Arkansas has no loading rates defined for secondary-treated effluent. Tyler [5] defined secondary-treated effluent loading rates based on organic loading of <30 mg•L −1 of biological oxygen demand (BOD). Wastewater with low BOD levels was hypothesized to reduce pore clogging at the trench-soil interface. With reduced bio-mat formation, soils could be hydraulically loaded with secondary-treated effluent at a rate greater than primary-treated effluent. Therefore, the focus of this study was to investigate the impact of hydraulically loading secondary-treated effluent into soils that are too limiting for a conventional wastewater system or an advanced treatment unit with drip disposal. This study also considered soils that are not limiting, but have a reduced disposal area. Exploring an alternative method for managing secondary-treated effluent disposal may reduce the need for an overland-flow NPDES permit, safely disperse secondary-treated effluent back into the hydrologic cycle, and provide development options as homeowners continue to move into rural areas that require an onsite wastewater system. It was hypothesized that limiting soils hydraulically loaded at two times the loading rate defined by Tyler [5] with secondary-treated effluent will not exceed a ponding depth of 27 cm for a consecutive period greater than 14 days in any disposal trench. It was also hypothesized that the performance of a reduced shallow disposal field will differ over time, specifically between wet and dry seasons.

Site Description
Six individual homeowners were selected in 2016 within a 64-ha area of a single sub-division in Saline County, Arkansas ( Figure 1). The homeowners were willing cooperators whose lots were all within close proximity to one another and on similar, limiting soils for a traditional drainfield that ranged in size from 1.2 to 4.8-ha. The homes had between three and four bedrooms and had between two and six occupants throughout the duration of this study. The study area, and six homes within the study area, resides in the thermic soil temperature regime within the Ouachita Mountains, Major Land Resource Area (MLRA) 119 [6]. The mean annual air temperature in the region is 17˚C, while the mean annual precipitation ranges between 122 and 140 cm [7]. Within the research sites, the soils are typically shallow to weathered shale and have argillic soil horizons that begin between 30 and 36 cm from the surface, where both shallow bedrock and argillic horizon presence can restrict water flow through the soil profile.
For four of the six sites, the soils are mapped as a Carnasaw-Townley association (fine, mixed, semiactive, thermic, Typic Hapludults) with no mounds present. Based on visual assessment, the soils present at these four sites had shallow seasonal water tables, as evidenced by redoximorphic depletions present to the soil surface. At the remaining two sites, the soils are mapped as a Caddo-Messer complex (fine-silty, siliceous, semi-active, thermic, Typic Glossaqualfs) with mounds present. Based on visual observation, the soils between the mounds had shallow seasonal water tables, as evidenced by redoximorphic depletions present to the soil surface, while the soils associated within the mounds had seasonal water tables evident by redoximorphic features beginning at approximately the 55-cm depth from the soil surface. However, based on the estimated volume of household wastewater produced at the two mounded sites, the  [2], and based on redoximorphic features present, the area associated with the mounds was inadequate for a conventional disposal area. Therefore, all six sites had limiting soils due to shallow water tables (four sites) and/or had insufficient area of suitable soil (two sites) for a conventional wastewater system. As an alternative to the conventional wastewater system (i.e. septic tank, distribution box, and a disposal field), the Rules and Regulations for onsite wastewater disposal in Arkansas [2] allow sites with limiting soils or disposal areas to utilize an advanced treatment unit to renovate household wastewater before discharging to an overland-flow point on the soil surface. All six research sites utilized an advanced treatment unit manufactured by Orenco Systems (Model AX20-RT mode 1B, Sutherlin, OR) or Bio-Microbics, Inc. (Model MicoFAST 0.5, Lenexa, KS). Both types of units consisted of a settling compartment, a secondary-treatment compartment, and final compartment for ultraviolet (UV) disinfection and sampling. The advanced treatment units produce a quality of wastewater that is acceptable to discharge onto the soil surface, which is effluent containing 10 mg•L −1 or less biochemical oxygen demand (BOD), 15 mg•L −1 or less total suspended solids (TSS), 6 mg•L −1 or greater dissolved oxygen (DO), and a pH between 6.0 and 9.0 [9]. Consequently, each landowner in this study has a NPDES permit to surface-discharge their household wastewater after passing through the advanced treatment unit. Homeowners agreed to allow an experimental shallow-drain-field system to be installed on their property and to be studied to potentially find an alternative disposal method to surface discharging.

Treatments and Experimental Design
Among the six sites, two sites had experimental shallow-drain-field systems installed into the mounds that were present, while the other four sites, which had no mounds, had experimental shallow-drain-field systems installed on contour with the natural slope. Secondary-treated effluent loading rates were derived based on the soil texture at the most-limiting layer with guidance from previous loading rates derived for secondary-treated effluent [3]. The initial loading rates for the non-mounded (12.2 L•m −2 •d −1 ) and mounded sites (32.5 L•m −2 •d −1 ) were doubled for both the non-mounded and mounded disposal sites (24.4 and 65.0 L•m −2 •d −1 , respectively). The six sites had similar site characteristics, including similar soil map units, soil profile textures, slopes, landscape positions, and other soil morphological characteristics (Table 1). area. Once an acceptable disposal area location was determined, one soil pit per site was excavated to an approximate depth of 120 cm in each of the defined disposal areas to evaluate the soil profile characteristics and establish a hydraulic loading rate. Soil descriptions were prepared for each horizon to a depth of 120 cm, recording information such as the texture by feel, estimated coarse fragment concentration (estimated to be 40%) in the upper 30 cm, moist matrix Munsell color, and redoximorphic feature (i.e. concentrations and depletions) presence and abundance. Representative soil samples were collected from each horizon for soil particle-size analyses using a modified 12-hr hydrometer method [10] after oven drying for 48 hours at 70˚C and grinding and sieving sub-samples through a 2-mm screen. Four disposal trenches were excavated at each site with a rubber-tracked 4500 kg track hoe. Disposal trenches were 21-m long, 35-cm deep, 45-cm wide, spaced 1.2 m center to center (Figure 2(a)), and disposal trenches were installed following the contour (i.e. the same elevation on the trench bottom along the length of a trench). Disposal trenches were backfilled with a 20-cm thick bed of commercially available, washed, crushed gravel, 2.5 cm in diameter or less. A low-pressure distribution network was constructed from 1.9-cm-diameter polyvinyl chloride (PVC), Schedule-40 pipe and installed in the center of each disposal trench atop the crushed-gravel bed (Figure 2(b)). Holes, 0.32 cm in diameter, were drilled every 120 cm and protective orifice shields (STF-106TDS, SIM/TECH, Boyne City, MI) were snapped over each hole. Geotextile fabric (2624RB 24 × 300, Advanced Drainage Systems, Hilliard, OH) was cut to a 45-cm width and laid over the low-pressure distribution network and gravel bed. The gravel and pipes were then covered with 15 cm of native soil, with slight mounding over the disposal trench to allow for settling over time.   Image of an in-trench monitoring port being installed at Site B (b). Site B was one of the mound disposal areas. The inspection port is used to measure ponding depths within the disposal trench. Each trench has its own monitoring port.
allowed for the soil solution to equilibrate inside the monitoring port so the depth from the soil surface to free solution (i.e. solution ponding) inside the trench could be measured (Figure 3(b)).
An observation port, also consisting of 8.9-cm-diameter PVC pipe, was installed vertically to a depth of 60 cm approximately 1.5 m up-slope from the disposal area to allow for observation and measurement of the seasonal water table. Four slits, approximately 0.3-cm wide and 20-cm long, were cut vertically from the bottom up. The slits allowed free water to flow into the observation port to facilitate measurement of the depth to free water from the soil surface.
When construction of each site was complete, the surface was manually seeded with a rye (Lolium spp.)-Bermudagrass (Cynodon dactylon) mixture at an approximate rate of 180-kg seed ha −1 and the seeded area was covered with straw to prevent erosion.
Each research site was connected to the homeowner's advanced treatment unit. Soil texture, determined during initial assessment of the disposal area, was used to set an expected effluent loading rate. The flow coming into the disposal site was recorded by reading the flow meter between observations and minor changes were made in the first month of the study by diverting excess flows or by turning off disposal trenches due to inadequate flows. Once the target effluent loading rate was achieved, no adjustments were made for the remainder of the study.

Effluent Source and Characterization
The secondary-treated effluent used to hydraulically load the shallow-disposal Thus, each research site had secondary-treated effluent delivered in small, timed doses (Table 2), evenly distributed to the disposal site by the low-pressure distribution network.
Requiring the limiting soil profile to accept and transport primary-treated effluent may have complicated the study by the formation of a biomat or possible surfacing during the study causing an environmental concern. For this reason, the secondary-treated effluent was sampled and characterized every six months.

Disposal Site Monitoring
Monitoring of the disposal areas consisted of recording flows into each disposal area, the depth to free solution in each disposal trench, the depth to free water in the observation well, overall site conditions, and rainfall amounts. Disposal-site monitoring occurred at 14-day intervals over the 14-month research period (i.e. January 2017 to March 2018). Flows at each disposal site were recorded at the flow meter. The reading was recorded in written format and a digital picture was taken. Flows were compared against public water bill usage, which confirmed measured flows to each disposal site were reasonable. A hand-held tape measure was used to record ponding depths in the disposal trenches. Measurements were made from the downslope lip of the in-trench monitoring port to the top of the ponded-solution surface, if present. A tape measure was also used to record the Rainfall data recorded at Site A were compared to rainfall data recorded within the research area that was publicly available through the Farm Logs web application [11]. The Farm Logs rainfall history and tracking came from a dataset that the National Oceanic and Atmospheric Administration (NOAA) produced. The NOAA sources data from multiple radar and ground stations within the county to algorithmically calculate the amount of precipitation that falls on a high-resolution grid across the continental United States. The NOAA factors in variables like wind and terrain that influence where the rain actually hits the ground, which was done within 1 km (0.6 mi) of accuracy [12]. Rainfall amounts reported through the Farm Logs application were determined to be accurate when compared to the actual on-site measurements. Farm Logs rainfall data were used for the remainder of the study period after January 2017.

Disposal Site Failure Criteria
Disposal site failure criteria have previously been based on the presence of a certain amount of solution storage in a trench for an extended period of time [13] [14], as the disposal field trench is designed to facilitate dispersal of effluent into the soil rather than for storage. Based on guidance from several previous reports [13] [14] [15], for the purposes of this study, if any disposal trench in a disposal area had a solution ponding depth in excess of 27 cm, which was 8 cm from the soil surface, for two or more consecutive 14-day measurement intervals, the disposal trench was noted as an exceedance.

Data Analyses
In-trench and observation well ponding depths were plotted over time. Temporal trends in in-trench ponding depths among active, effluent-receiving lines at each site were visually assessed relative to the soil surface, depth of the gravel, and depth of the bottom of the trench and for the frequency of in-trench ponding exceeding the depth to the in-trench gravel. Temporal trends of ponding depths in the observation wells were also visually assessed. In addition to visually assessing temporal trends in ponding depths, analysis of variance was conducted separately by site, using all temporal measurements as replications, using Minitab (version 13.31, Minitab, Inc., State College, PA) to evaluate differences in mean ponding depths over time among trench lines. Similar to Prater et al. [16] and Gibbons et al. [15], linear regression analyses were also conducted using Minitab, separately by trench line, to formally evaluate the temporal trend in ponding depths over time (i.e. whether ponding depths were increasing, de-

Effluent Characteristics
A prerequisite to studying a reduced drainfield in a limiting soil profile was to utilize secondary-treated effluent capable of meeting minimum overland-flow discharge requirements as defined by the Arkansas Department of Environmental Quality [9]. The basic premise of using secondary-treated effluent in the study was to minimize the formation of a biomat by managing effluent with low

Rainfall Characteristics
Rainfall during the study period had a direct impact on the performance of the     An interceptor drain installed up-slope from the disposal area could be used to divert the seasonally shallow water table and may have alleviated a portion of the hydrologic stress to the disposal area at each site.

Peak Flows
Each non-mounded disposal site studied had existing infrastructure in place that

Size of Disposal Areas
The non-mounded disposal areas that received secondary-treated effluent covered 78 m 2 , where each site had four lines that were 21.3-m long on 1.2-m centers. The redoximorphic features of the soil in the non-mounded sites had no corresponding loading rate in the Arkansas Rules and Regulations [2] to compare a similar disposal area using primary-treated effluent. However, if a loading rate of 8.4 L•m −2 •d −1 [5] was assumed based on the soil texture of the most-limiting horizon (i.e. a clay-textured horizon at some relatively shallow depth at all sites),  No ponding was recorded in any trench during this period at Site D; however, the seasonal water table was present throughout the study period. and a standard trench spacing of 2.4-m was used, the disposal area required would have been 372-m 2 using primary-treated effluent. Utilizing secondary-treated effluent in a reduced disposal area that occupied only 21% of the area required for primary-treated effluent was a significant area reduction when contemplating an alternative method to disposing of secondary-treated effluent other than by overland-flow surface discharging or when the suitable area for disposal is greatly limited.  Site B and C are not represented because no seasonal water table was measured, nor exceedance recorded during the study for these two sites.

Hydraulic Loading
The accepted flow for a three-bedroom home per the Arkansas Rules and Regulations is 1400 L•d −1 [2]. The four non-mounded sites had average daily flows > 454 to <1749 L•d −1 ( Table 3). The objective of the study was to load each disposal site at a minimum of two times the loading defined by Tyler [5] for secondary-treated effluent. Flows were recorded and adjustments were made from January through February 2017 to meet the minimum objective. Site A had excess flow due to additional infiltration and inflow of climatic water. Site D measured minimal flows from the home and did not have an exceedance or ponding depth to record in any trench, except for one occurrence on December 23, 2017 when a ponding depth was measured after a 7.6-cm rain the night before. However, the ponded water was no longer evident in the disposal trench three days later. Site E and F had expected flows for a three-bedroom home of 697 to 768 L•d −1 . The four non-mounded sites achieved >2, but <3.8 times the accepted loading rate for secondary-treated effluent (Table 3).

Lateral Movement
Due to the 2% to 4% slope of the disposal areas at all four research sites, sub-surface lateral movement of secondary-treated effluent between trenches was expected and evolved over the study period. From January through February where mean ponding depth averaged 0.9 cm across all four lines throughout the entire study (Table 4). At Site A, mean in-trench ponding depth increased by a factor of 1.8 from lines 1 and 2, which did not differ, to lines 3 and 4, which did not differ. At Sites E and F, mean ponding depth increased 6-and nearly 2-fold, respectively, from line 1 to lines 2 and 3, which did not differ. Understanding the lateral, sub-surface flow phenomenon may allow a designer to hydraulically load the upslope-most disposal trench with more secondary-treated effluent and, by the same logic, hydraulically load the more downslope trenches with less secondary-treated effluent and expect effluent renovation from sub-surface, lateral movement of effluent between trenches.
Among the four non-mounded sites, none of the four lines at any site had ponding depths that changed over time throughout the duration of the study, except for line 4 at Site F, which significantly decreased over time (P = 0.01; Table 5). These results indicate that, at least within the first 18 months after initial dosing, the secondary-treated effluent was of sufficient quality to minimize biomat formation which is often cited as the reason for absorption field failure when using primary-treated effluent.     table in a limiting soil profile and its impact on a disposal area was not part of this study;   however, the observation could be made that, if a soil profile was limiting due to   a shallow seasonal water table and the seasonal water table was diverted around the disposal area using an interceptor drain, the efficiency of the disposal area in the disposal area would improve.

Ponding Depths for Site A vs. Site D
Sites A and D were located on the same landscape position (backslope), had the same slope (3%), and similar soil profile characteristics, with the minor exception of the depth to the textural change (Table 1) Based on the data collected, textural changes in the profile should be taken into consideration when contemplating disposal of secondary-treated effluent in a reduced disposal area in profile-limiting soils, which should apply to any disposal site, regardless of using primary or secondary-treated effluent. Site D used only one of the four disposal lines during the study (i.e. line 1). Although Sites D, E, and F had at least one disposal line turned off, ponding depths were still recorded for each of the four-disposal lines. Site A always had a measurable ponding depth in the lower disposal trench (Figure 11(a)). Sites E ( Figure 11(b)) and F (Figure 12(a)) had measurable ponding depths in line 4.

Visual Changes in Vegetation
Site D did not have any measurable ponding to record for the duration of the study, with exception of after a single rain event in December 2017 ( Figure  12(b)). Observing the ponding depths in the lower disposal lines, when secondary-treated effluent was not delivered to the lowest disposal line, clearly demonstrated the sub-surface, lateral movement of the secondary-treated effluent downslope. Based on the data, the concept of sub-surface, lateral flow should be considered in the design criteria when using a reduced disposal area and secondary-treated effluent in profile-limiting soils, as sub-surface, lateral movement of effluent would still provide renovation despite the effluent's minimal vertical flow in the soil profile.

Size of Disposal Areas
The mounded disposal areas that received secondary-treated effluent covered 60

Hydraulic Loading
The two mounded sites had average daily flows that ranged from >416 to <1703 L•d −1 (Table 4), whereas the accepted, estimated flow for a three-bedroom home is 1400 L•d −1 [2]. Neither site B nor C had enough flow to measure an exceedance or ponding depth in any of the disposal lines throughout the entire study. The two mounded sites achieved >2.5, but <3.8 times the accepted loading rate for secondary-treated effluent (Table 5).

Visual Changes in Vegetation
In contrast to the non-mounded sites, changes in landscape vegetation within the mounded disposal areas, or downslope from the mounded disposal areas, were negligible. Sites B and C did not show any changes in vegetation within or downslope of the disposal area, which corresponded with the lack of any ponding-depth exceedances recorded during the study.

Ponding Depths in Line 4 When Line 4 Was Turned off
Similar to the non-mounded sites, during the initial measurement period from September 2016 to February 2017, the number of disposal trenches utilized during the study were adjusted to reflect a loading rate at a minimum of two times the accepted loading rate for secondary-treated effluent. Sites B and C used only one of the four disposal lines during the study (i.e. line 1). Though Site B had significantly greater mean ponding in line 4 than in the other three lines, the mean ponding depth was only 0.7 cm (Table 4). Similarly, ponding depth did not differ among lines at Site C and averaged only 0.7 cm per line throughout the duration of the study (Table 4). Thus, Sites B and C did not have any appreciable effluent/water ponding for the duration of the study in any of the lower disposal trenches beyond line 1. Consequently, in contrast to the non-mounded sites, lateral movement of secondary-treated was not evident at Sites B or C throughout the study.
Similar to most of the non-mounded sites, ponding depth in the lines at the two mounded sites did not change over time ( Table 5). The lack of an increase in ponding depth over time lends credibility to the feasibility of using secondary-treated effluent in a shallow drain field with reduced area.

Implications
If managed properly, secondary-treated effluent disposed in limiting soils or reduced disposal areas can be considered an alternative for disposal sites with limiting soils that previously were deemed unsuitable. Disposing of wastewater back into the soil profile versus an overland-flow discharge protects the environment by utilizing the soil as the final destination (i.e. hydrologic cycle), reduces the regulatory burden and compliance challenges with surface discharges, and is the responsible way to manage the wastewater.
Throughout the study, the impact from fluctuating seasonal water tables was a

Conclusions
Managing wastewater in rural settings is becoming a challenge as Arkansans' move into areas where conventional wastewater systems are not feasible. When considering an alternative to a conventional on-site wastewater system, there are limited empirical data supporting disposing of secondary-treated effluent in limiting soils other than by overland-flow surface discharge. Results showed that soils that may be unsuitable for a conventional on-site wastewater system may be suitable using secondary-treated effluent with a shallow or a reduced disposal area.
Based on the absence of appreciable secondary-treated effluent ponding at Sites B, C, and D during the study and the minimal exceedances in Site A, E, and F, which was linked directly to fluctuating seasonal water tables, it is reasonable to consider hydraulically loading secondary-treated effluent at a rate Tyler [5] established based on soil textures and structure. Consideration must be given to hydraulically loading secondary-treated effluent in unsuitable soils or suitable soils with a reduced disposal area.