Existing evidence related to soil retention of phosphorus from on-site wastewater treatment systems in boreal and temperate climate zones: a systematic map

Abstract

Background: In Sweden there are nearly one million soil-based on-site wastewater treatment systems (OWTSs). OWTSs may contribute to eutrophication of surface waters, due to the discharge of phosphorus (P). Hence, in certain cases, a high P removal rate (up to 90%) of OWTSs is required by Swedish authorities. Since these requirements may have costly consequences to property owners, it is debated whether they are too strict. In this debate, it is often claimed that the soil retention of P occurring in the natural environments may be underestimated by authorities. Soil retention is the inhibition of the transport of P through the ground, due to different chemical, physical and biological processes occurring there. These processes make the P transport slower, which may reduce the unwanted impact on receiving water bodies. However, the efficiency of soil retention of P remains unclear. The objective of this systematic map was to collect, code, organise and elucidate the relevant evidence related to the topic, to be able to guide stakeholders through the evidence base, and to support future research synthesising, commissioning, and funding. The systematic map was carried out in response to needs declared by the Swedish Agency for Marine and Water Management but the conclusions should be valid for a wider range of countries across boreo-temperate regions.

Methods: Searches were made for peer-reviewed and grey literature using bibliographic databases, search engines, specialist websites, and stakeholder contacts. The references were screened for relevance according to a predefined set of eligibility criteria. A detailed database of the relevant studies was compiled. Data and metadata that enable evaluation and discussion of the character and quality of the evidence base were extracted and coded. Special focus was placed on assessing if existing evidence could contribute to policy and practice decision making. Descriptive information about the evidence base was presented in tables and figures. An interactive evidence atlas and a choropleth were created, displaying the locations of all studies.

Review findings: 234 articles out of 10,797 screened records fulfilled the eligibility criteria. These articles contain 256 studies, performed in the field or in the laboratory. Six different study types were identified, based on where the measurements were conducted. Most studies, including laboratory studies, lack replicates. Most field studies are observational case studies.

Conclusions: It is not possible to derive valid generic measures of the efficiency of soil retention of P occurring in the natural soil environment from available research. Neither does the evidence base allow for answering the question of the magnitude of the potential impact of OWTSs on the P concentration in recipients on a general basis, or under what conditions OWTSs generally have such an impact. A compilation of groundwater studies may provide examples of how far the P may reach in x years, but the number of groundwater studies is insufficient to draw any general conclusions, given the complexity and variability of the systems. Future research should strive for replicated study designs, more elaborate reporting, and the establishment of a reporting standard.

Keywords: Adsorption; Drainfields; Eutrophication; Infiltration; OWS; OWTS; Phosphorus removal; Precipitation; Septic systems; Septic tanks; Soil treatment; Sweden.

Fig. 1

Conceptual model of the natural soil system where P retention is considered, and the main P retention processes in this system

Fig. 2

Flow diagram showing the screening process [23]

Fig. 3

Distribution of types of publications

Fig. 4

Numbers of included articles published each respective year during the period 1961 to 2021

Fig. 5

Number of articles from each represented country

Fig. 6

Choropleth displaying the geographical distribution of studies

Fig. 7

Snapshot of evidence atlas displaying the geographical distribution of field studies. The two studies placed in the Atlantic Ocean lack coordinates. As regards the yellow and green dots with numbers, the colours do not indicate a specific study type. Yellow dots with numbers contain more than 10 studies that appear while clicking on it in the html-based version; green dots contain 2–9 studies that appear while clicking on it. (Yellow dots without a number indicate a single surface water study.)

Fig. 8

Number of field studies performed in each represented climate zone. ND: no data reported or inferable

Fig. 9

Categorisation of included studies. The numbers in brackets refer to numbers of studies

Fig. 10

The red ellipse shows which part of the system that is in focus in studies performed in the vadose zone, within facility. The red, dashed line indicates the limits of the complete natural soil system between facility and recipient, that is, the system of primary interest

Fig. 11

Number of studied facilities within five size categories. ND: no data reported

Fig. 12

Duration of wastewater exposure. ND: no data reported

Fig. 13

The red ellipse shows which part of the system that is in focus in studies performed in the vadose zone, in the natural soil environment. The red, dashed line indicates the limits of the complete natural soil system between facility and recipient, that is, the system of primary interest

Fig. 14

Duration of wastewater exposure. ND: no data reported

Fig. 15

The red ellipse shows which part of the system that is in focus in studies performed in the groundwater zone. The red, dashed line indicates the limits of the complete natural soil system, that is, the system of primary interest

Fig. 16

Number of studied facilities per age group (years of wastewater exposure). ND: no data reported

Fig. 17

The red ellipse shows which part of the system that is in focus in studies performed in surface water. The red, dashed line indicates the limits of the complete natural soil system, that is, the system of primary interest

Fig. 18

Number of studies using respective method to evaluate impact of OWTS on surface water. Several methods may have been used in the same study

Fig. 19

Number of studies using columns with diameters within given ranges. ND: no data reported. (Three studies, using boxes instead of columns, are not included in the diagram.)

Fig. 20

Number of studies using columns with soil package heights within given ranges (cm). Only the largest height in studies where several heights were used is included in the diagram. The ranges indicate flow lengths in most cases. However, this is not completely consistent for the following reasons: (1) it is not always clear whether package height or column height is reported, (2) sometimes there are ports at different heights which means that the flow length is shorter than package heights. ND: no data reported. (Three studies, using boxes instead of columns, are not included in the diagram.)

Fig. 21

Duration of wastewater flow. If more than one duration was applied within the same study, all durations are included in the diagram. Hence, the sum of studies is greater than the number of included column studies. ND: no data reported

Fig. 22

The number of cases using soil of respective soil order. ND: no data reported