Abstract
Although groundwater quality depends on microbial processes in the soil treatment area (STA) of onsite wastewater treatment systems (OWTS), our understanding of the development of these microbial communities is limited. We examined the bacterial communities of sand, sandy loam, and clay STAs at different depths in response to septic tank effluent (STE) addition using mesocosms. Terminal restriction fragment length polymorphism (TRFLP) analysis was used to compare the bacterial community structure and composition of STE, native soil prior to STE addition (UNX) and soil exposed to STE (EXP). Principal component analysis separated communities with depth in sand but not in sandy loam or clay. Indices of richness, diversity, and evenness followed the order: sandy loam > sand > clay. Analysis of TRF peaks indicated that STE contributed least to the composition of STA bacterial communities (5%–16%), followed by UNX soil (18%–48%), with the highest proportion of the community made up of TRFs not detected previously in either UNX or STE (50%–82%) for all three soils. Soil type and depth can have a marked effect on the structure and composition of STA bacterial communities, and on the relative contribution of native soil and STE to these communities.
Highlights
Onsite wastewater treatment systems (OWTS) rely on biogeochemical processes and ecological interactions that take place in the soil treatment area (STA; known as drainfield or leachfield) to renovate wastewater, many of which are driven by microorganisms
Principal component analysis (PCA) based on the presence or absence of terminal restriction fragments (TRFs) shows that the bacterial communities of unexposed sand (UNX) and septic tank effluent (STE) differs from sand exposed to STE (EXP) (Figure 1)
Our results show that soil type can play an important role in determining the size, composition, and structure of bacterial communities, and in the relative contribution of dominant soil and STE
Summary
Onsite wastewater treatment systems (OWTS) rely on biogeochemical processes and ecological interactions that take place in the soil treatment area (STA; known as drainfield or leachfield) to renovate wastewater, many of which are driven by microorganisms. Zhou et al [7] examined two sandy soils from the surface (0.05 m) and vadose (1.57 m) zones and found microbial diversity to decrease with depth These studies suggest that the depth at which an STA is built may be an important determinant of the composition of the resulting bacterial community. An understanding of the factors that control the composition and structure of the bacterial community of STAs is important for developing our ability to design systems that renovate water quality in a predictable manner and adjusting system conditions to promote desirable processes To this end, we conducted a mesocosm-scale experiment to (i) evaluate the structure and composition of the bacterial communities of STAs as a function of soil texture and depth below the infiltrative surface; and (ii) to determine the relative contribution of STE and native soil to these bacterial communities. These data, in conjunction with TRFLP data for STE and native soil unexposed to STE, were used to examine the relative contribution of native soil and STE to the resulting STA bacterial communities
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