ON-SITE WASTEWATER SYSTEM CLEARANCE (SET-BACK) and SEPARATION DISTANCES - A REVIEW

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1 ON-SITE WASTEWATER SYSTEM CLEARANCE (SET-BACK) and SEPARATION DISTANCES - A REVIEW Compiled By: Ian Gunn, Editor, On-Site NewZ, July Background This review has been prompted by an enquiry related to the scientific basis for setting clearances to groundwater (vertical clearance) and to water supply wells and surface waters (horizontal clearances) under regulatory rules and/or requirements. 2.0 US Public Health Service Guidelines 1957 The horizontal and vertical clearances set out in the rules of various agencies have been based on arbitrary values which appear to have their origins in the US Public Health Service (USPHS) studies initiated in A five year programme of investigations was undertaken in order to develop a factual basis on which (septic-tank-soilabsorption systems) could be designed, installed, and maintained [Ref. 1]. The 1957 Manual of Septic-Tank Practice incorporated the results of these studies (which were described in three technical reports) together with information gained from other studies and field experience. The manual sets out the following in relation to clearances: Vertical Clearance: The maximum elevation of the ground-water table should be at least 4 feet below the surface. Rock formations or other impervious strata should be at a depth greater than 4 feet (1220 mm) below the bottom of the trench or seepage pit. Unless these conditions are satisfied, the site is unsuitable for a subsurface sewagedisposal system, except for isolated systems. Horizontal Clearance: The selection of the leaching system will be dependant to some extent on the location of the system in the area under consideration. A safe distance should be maintained between the site and any source of water supply. Since the distance that the pollution will travel underground depends upon numerous factors, including the characteristics of the subsoil formations and the quantity of sewage discharged, it is not practical to specify minimum distances that would be reasonable in all localities. Ordinarily, of course, the greater the distance, the greater will be the safety provided. In general, all subsurface absorption systems should be kept 100 feet (30 m) from any water-supply well, feet (15 m) from any stream or watercourse, and 10 feet (3 m) from dwellings or property lines. Seepage pits should not be used in areas where domestic water supplies are obtained from shallow wells, or where there are limestone formations and sinkholes with connection to underground channels through which pollution may travel to water sources. Seepage pits should not be located within 20 feet (6 m) of any dwelling or property line. Details pertaining to local water wells, such as depth, type of construction, vertical zone of influence, etc., together with data on the geological formations and porosity of subsoil strata, should be considered in determining the safe allowable distance between wells and subsurface disposal systems. Septic tanks should be located where they cannot cause contamination of any well, spring, or other source of water supply. --- Tanks should never be closer than feet (15 m) from any source of water supply; and greater distances are preferred where possible. The septic tank should not be located within 5 feet (1.5 m) of any building. Comment: The basis of the recommended clearances appears to be fairly arbitrary, and these are offered as general guidelines in the absence of detailed site information. Effectively the manual is saying that the greater the clearance the higher the factor of safety, and that actual clearances can be set on a site and system specific basis. It also sets clearances for septic tanks at half the value as for soil absorption systems. With respect to the clearance to groundwater, the 4 feet (1220 mm) surface to water table depth requirement and the 18 to 24 inch (460 to 610 mm) trench depth range allowed for in the manual means clearance of 30 to 24 inches (760 to 610 mm). This represents the depth of un-saturated soil available to provide treatment of the septic tank effluent discharge to the soakage trench system. 3.0 US Public Health Service Guidelines 1967 (Revision) The 1957 original document was reprinted in 1963, and then revised in 1967 [Ref. 2]. Vertical Clearance: The significant change from 1957 is that the 4 feet (1220 mm) clearance to groundwater was changed to become the clearance below the base of trench or seepage pit (and not to the ground surface). This increases the available soil depth for treatment of the discharged effluent to 48 inches (1220 mm) regardless of the depth of trench. Horizontal Clearance: The clearance for the tank from wells and buildings remained unchanged. While the preamble to the horizontal clearances for soil absorption systems remained the same, more detailed information was given in tabular form for On-Site NewZ Setbacks and Clearances (Historical overview) [23 January 2014] Page 1

2 minimum distances as distinct from the in general distances of The tabulated values are as follows: Minimum distance between components of sewage disposal system Element of System Building sewer Septic tank Disposal field and Seepage Bed Seepage Horizontal Distance - feet (metres) Well or Water Stream House Site suction supply Bound line line ary 100 (30.5) 100 Pit (30.5) Cesspool 1 (45.7) 10 (3) 10 (3) 25 (7.6) (1.5) 20 (6.1) 20 (6.1) 20 (6.1) 10 (3) 5 (1.5) 10 (3) 15 (4.6) Comment: The changes in 1967 can be interpreted as a more prescriptive approach. In respect of the vertical clearances, the depth to groundwater has been increased, and the reference to a waiver for isolated systems has been removed. For horizontal clearances, there can be a tendency for users to apply the numbers in the table without reading the qualification in the paragraph that follows. This qualification implies that each case should be considered relative to local circumstances. In some cases this could mean that clearances should be increased in other cases it could mean an opportunity to reduce the clearances (such as in remote locations with minimal environmental /public health risk). 4.0 NZ CP 44:1961 Code of Recommended Practice This code [Ref. 3] for the Disposal of Effluent from Household Septic Tanks was based on the USPHS Publication No. 526 of It set horizontal clearances at the same values as in the US. These were: 100 feet (30 m) to water supply wells, feet (15 m) to streams, 10 feet (3 m) to dwellings, and 5 feet (1.5 m) to property boundaries. The clearances to dwellings and property boundaries were mandated by the Drainage and Plumbing Regulations of Surprisingly, although the section discussing the suitability of the soil for disposal of effluent commenced with similar wording to that in Publication No. 526, there was no carry through discussion on depth to groundwater. Hence, the Code does not set any groundwater clearances. 5.0 US EPA Design Manual 1980 Onsite Wastewater Treatment and Disposal Systems [Ref. 4] provided the on-site wastewater servicing industry with a design manual containing a comprehensive overview of systems and techniques for handling domestic and institutional flows in un-sewered areas. Clearances: Section refers to US Soil Conservation Service (SCS) limitations ratings which are then summarised in Table 3-2. These are based on a soil absorption system with the bottom surface located 2 ft (0.6 m) below the soil surface. In many cases the limitations can be overcome through proper design. Therefore, the interpretations should be used only as a guide. The table sets out the following soil limitation ratings: Site Property Depth to bedrock or hardpan Depth to high water table below ground surface Soil Limitation Slight Moderate Severe > 6 ft 3.3 to 6 ft < 3.3 ft (1.8 m) (1 to 1.8 m) (1 m) > 6 ft (1.8 m) 4 to 6 ft (1.2 to 1.8 m) < 4 ft (1.2 m) Site criteria for trench and bed systems are set out in section and Table 7.1 as ranges of typical values. These are as below. Item Typical Horizontal Separation Distances Water supply wells Surface waters, springs Escarpments, cuts Property boundary Unsaturated Depth Depth to Bedrock Criteria to 100 ft (15 to 30 m) to 100 ft (15 to 30 m) 10 to 20 ft (3 to 6 m) 5 to 10 ft (1.5 to 3 m) 2 to 4 ft (610 to 1220 mm) of unsaturated soil between bottom of system and seasonal high water table. 2 to 4 ft (610 to 1220 mm) of unsaturated soil between bottom of system and bedrock The notes to the above table state that the horizontal clearances are intended only as a guide. Safe distance varies from site to site, based on topography, soil permeability, groundwater gradients, geology, etc. The manual then goes on to provide guidance on the use of mound systems. Horizontal distances are the same as in the table for trenches and beds, but groundwater clearances are significantly lower. Indeed, the mound system was developed specifically for situations where water table height limitations prevented the use of trenches and beds. The depth On-Site NewZ Setbacks and Clearances (Historical overview) [23 January 2014] Page 2

3 of unsaturated soil between the original soil surface (mound base) and seasonally high groundwater (or creviced bedrock) is to be around 510 to 610 mm. This compares with 1220 to 1800 mm for trenches/beds (based on constructed depths of 610 mm). Comment: The manual takes a site specific approach to setting clearances based on environmental and soil conditions. This clearly requires a detailed site assessment and specific design approach to suit the available or desired clearances. 6.0 NZ Standard NZS 4610:1982 The publication of this Standard [Ref.5] for Household Septic Tank Systems took a new approach to assessing soil suitability for accommodating effluent disposal fields. It noted that the horizontal clearances for the septic tank to buildings and property as set out in CP 44:1961 remained the same under the 1978 Drainage and Plumbing Regulations. However, no specific vertical or horizontal clearances were set. The Standard has the following to say on both these (the italicised portions below are quotes from the Commentary paragraphs associated with the substantive clauses in the Standard): Siting of septic tanks and effluent disposal systems: Clause The disposal field should not be installed in any location where any well or watercourse used as a source of public supply may become polluted or contaminated unless special design precautions to the approval of the Engineer and the Regional Water Board are taken to prevent such pollution or contamination. Commentary: Specific clearances of wells, watercourses and other sources of water supply are to be set according to local circumstances and in conjunction with regulatory authorities. The adoption of arbitrary limits (for example 30 m to any water well, 15 m to any stream as in the superseded Standard CP 44:1961) is not considered appropriate as design methods are available to enable local decisions in respect of such clearances based upon local site conditions. Before approving an area for subdivision where septic tank or other on-site systems of household wastewater disposal are proposed, the local authority should require the determination of groundwater and surface water movements, and the influence such movements may have on the siting and design of on-site wastewater disposal installations, and the effect such installations may have on subsequent use of both ground and surface waters for water supply or other purposes. Soil qualities for effective disposal: Clause 6.1 Effluent disposal systems should be located on soils having satisfactory internal drainage, adequate depth and moderate slope, and be sited where the influence of surface and ground waters on the soils ability to accept effluent can be controlled. The potential effects of the disposal systems on both the environment and public health should be assessed in relation to nearby streams, water supply sources and neighbouring disposal fields. Commentary: Effluent will flow more readily through structureless and granular soils; these soils are less effective in preventing nutrients and bacteria from entering the groundwater. Problems of eutrophication and public health are to be avoided on sandy and pumice soils by careful siting and design. Soil limitation categories: Table 2 places a Limitation Category E Special Constraint on any soil situation where the soil is subject to high water tables, and where seepage paths could contaminate adjacent surface or groundwaters. Table 3 requires that the disposal field (is) to be based upon a full engineering investigation and special design procedures aimed at achieving best practical means of effluent disposal to meet all relevant public health and environmental requirements, and that modifications to deal with Category E conditions of high groundwater or seepage paths be implemented to deal with the circumstances of each particular situation. Comment: The 1982 Standard reverted to the initial position taken by the USPHS in respect of setting clearances based on local circumstances. The effect of this approach is that designers and regulators have to have a good understanding of soil and site characteristics relative to the pretreatment capabilities of septic tanks and alternative systems, and the treatment achievable as the effluent is absorbed into the soil in the soakage system. A lot more information was available in 1982 to assist decision making of this sort than was available in the 1960s. However, the Standard was not seen as the place to detail it. In the Commentary to Clause 1, the Standard stated that the more detailed assessments outside the scope of the document would have to be undertaken through reference to a manual of engineering design practice. At the time NZS 4610:1982 was produced, the US EPA design manual [Ref. 4] was widely available. 7.0 TP 58 Design Manual 1994 The site assessment section of TP 58 [Ref. 6] requires consideration to be given to water table variation (need to determine the capacity of the site and an appropriate disposal method which maximises on-site renovation of effluent and minimises groundwater impacts). The text then continues in addition, clearances to wells, waterways, site boundaries, buildings and topographical discontinuities (e.g. embankments) On-Site NewZ Setbacks and Clearances (Historical overview) [23 January 2014] Page 3

4 should be assessed so that their influence on the design of an appropriate disposal system can be allowed for. In respect of groundwater clearance, the manual states: The distance between ground level and the seasonally high groundwater level should be assessed as to its influence on the design process and the selection of an appropriate disposal system. A clearance will be required between the base of the disposal area and the critical watertable level to satisfy one of two situations: where no public health or environmental constraints exist then the clearance must be adequate to ensure that hydraulic conductivity requirements are met (and no adverse groundwater mounding occurs); where such constraints are present (e.g. due to the use of groundwater for individual or community water supply) then the clearance must be such that when combined with quality control provided by the pre-treatment and disposal system, the further renovation of effluent during its percolation through the unsaturated soil layer between the disposal area and the groundwater surface is adequate to meet environmental and public health criteria. The manual continues that, if necessary the clearance can be artificially increased by using a mounded treatment and disposal system. Comment: TP 58 centres its response to clearances around the design process. This must be based on a full site and soil assessment, and the matching of system selection and design to the environmental conditions. 8.0 Joint Australia New Zealand Standard 2000 There are several relevant clauses that relate to protection of environment and public health. These include [Ref. 7]: Performance objectives: these require all public health and environmental requirements be complied with, and that surface and groundwater are not polluted. In addition, cumulative and adverse environmental effects are to be allowed for Potential cumulative environmental effects arising from the long-term use of onsite systems are to satisfy the performance objectives of the Standard and the requirements of the regulatory authority The designer is required to provide in the design details of the setbacks (clearances) The site-and-soil evaluation procedure is to collate (via desk study) relevant site information including set-back requirements. The site-and-soil check which follows involves a field inspection to identify site constraints related to topography, buildings and services, and to define the best available location for the land-application system (disposal area) Land-application systems shall be sited so that they do not affect, or are not affected by and comply with requirements for setback distances from buildings, property boundaries, retaining walls, embankments, swimming pools etc. (Setback distance requirements vary from one regulatory authority to another. There is no agreed setback or series of setback distances.) Land-application systems shall comply with the performance requirements set by this Standard and/or the relevant regulatory authority with respect to clearance from groundwater. Comment: The Standard did not attempt to preempt the design process and associated site and soil investigation by setting arbitrary horizontal and vertical clearances, as it recognised they are site specific, and may, in any case, be already set by regulatory authority requirements. 9.0 Manawatu-Wanganui Regional Council This Council is now known by the name horizons.mw. In November 2000 it published its regional guidelines for wastewater systems [Ref. 8]. Section states that it is widely accepted that a minimum depth of 600 mm good quality soil is needed below any soakage system to achieve adequate in-soil treatment of septic tank effluent. This depth ensures that bacteria and viruses remain trapped in the soil, during which many die, and do not seep through to the groundwater or near-by surface drainage systems. The document goes on to state that if the seasonal high groundwater table rises close to the property surface (i.e. saturates the soil in the top 600 mm) then conventional soakage systems such as trenches or beds are not suitable. In this situation the land application area will require: a 600 mm high mound of soil or sand; a reduction the effluent application rate (if the soil is saturated); encouragement of plant uptake of the effluent; and consideration of treating the effluent to a higher quality. Table 1 in section dealing with sizing the land application area provides suggested design sizing for low-pressure pipe (dosed loaded) trench systems for situations in varying soil types for groundwater conditions of either 300 mm depth below the distribution lines, 600 mm, and > 600mm. On-Site NewZ Setbacks and Clearances (Historical overview) [23 January 2014] Page 4

5 Section 5.15 explains that normal practice is to ensure that any part of the on-site system (edges of either septic tank or land application area) is located at least 3 m from any house or other living accommodation, and 1.5 m away from other buildings and property boundaries. On a sloping site a clearance of at least 10 m is to be provided to any downslope boundary (this space of 10 m forming a buffer zone ). Section indicates the buffer zone distance has to be extended to 20 m adjacent to any river, lake, natural wetland or artificial watercourse if it is wished to avoid having to apply for a Regional Council Resource Consent. This requirement is confirmed by the Regional Rules in Appendix 1. If the buffer clearance can be met, then the system is a permitted activity (able to be authorised by the District Council). A vertical clearance to groundwater of 0 mm is also required under the rule. Where these clearances cannot be met, then the project design is called in for Regional Council scrutiny and consenting via discharge permit Comment: horizons.mw effectively sets flexible guidelines similar to those in many Regional Council areas, within which clearances below certain limits can be accommodated by appropriate design procedures, and where the design can be called in for examination and issue of discharge permits. Such permits will be drawn up with conditions set to ensure that environmental and public health requirements are met. One notable omission from the horizons.mw list of clearances is any mention of that for water supply wells Wellington Regional Council Wellington Regional Council also does not specify minimum clearances to water supply wells or bores. The reason for this is set out in its guidelines for on-site wastewater systems [Ref. 9]. Section explains that the Regional Plan for discharges to land require a 20 m buffer distance between the soakage treatment area and any surface water body, farm drain, water supply race or the coastal marine area. If the on-site sewage system is in a water supply catchment, there has to be a m buffer distance. Soakage treatment areas inside these buffer zones require a discharge permit. The guidelines then go on to explain that: buffer distances from bores are not specified in the regional rules. This is because suitable distances are very site specific and depend on soil type, whether the bore penetrates a confined or unconfined aquifer, and whether the groundwater is flowing towards or away from the bore. Once contaminants in effluent reach groundwater, the groundwater can carry them hundreds of metres, so stipulating a separation distance from bores may provide a false sense of security to people drinking water from the bore. The easiest pathway for sewage effluent to contaminate groundwater is via poorly sealed bores. Properly designed soakage treatment areas and sealed bores should protect groundwater more effectively than buffer distances. Regarding depth to groundwater, the guidelines (section 5.2.3) explain that a minimum depth of 600 mm of good quality unsaturated soil below any soakage treatment area is a general precaution in this country. Measures such as separating the soakage area into several small elements, increasing its size (thus reducing the design loading rate), or using intermittent dosing, can all aid improved system performance under high groundwater conditions. Comment: The horizontal clearances to surface waters reflect a precautionary approach based on whether the on-site system is inside or outside a water supply catchment. Once again the clearances are for permitted activities under District Council consent procedures. Consent can be given by the Regional Council for systems within the clearances, in which case the Council will set appropriate conditions on the system design. The statement re clearances to water supply bores is the clearest statement so far as to reasons why a site specific approach is applicable in this area of design Revisions to TP TP 58 [Ref. 6] is currently undergoing revision. A review team set up by Auckland Regional Council is undertaking this project. Under consideration for inclusion in the manual is a detailed set of recommended minimum separation distances involving some 45 clearance values. Distances are set from buildings, property boundaries and embankments, and for surface water, water supply bore and groundwater. The water clearances cover separate values for each of two categories of soil characteristics (free draining to moderate soils, and moderate to slow draining soils). A clearance is set for each of five degrees of effluent pre-treatment prior to discharge to a land application system. Hence, there are 9 site conditions and 5 treatment categories, thus giving a total of 45 clearance values. The highest groundwater clearance proposed is 900 mm, the lowest 200 mm. Comment: This very prescriptive approach is provided to enable District Councils leeway to approve (without referral to a Regional Council) a wider range of site conditions and treatment scenarios than available in the past. It has to be borne in mind that the greater detail does not imply a high level of scientific input into setting the clearance values they are still arbitrary, although the proposed groundwater clearances are based to On-Site NewZ Setbacks and Clearances (Historical overview) [23 January 2014] Page 5

6 some extent on the findings of recent research reported in the overseas literature Research Findings Relevant to Clearances (a) On-Site NewZ Literature Review: Questions regarding the best information available in setting vertical clearances to groundwater have often been raised in New Zealand. An On-Site NewZ special report in 1997 [Ref.10] reviewed selected literature and assessed nine case studies. The final assessment in the review stated that: it is clear that passage of septic tank effluent through unsaturated soils ranging from moderately draining sandy loams to slowly draining loam clays has considerable capacity to remove bacteria in relatively short travel distances, i.e. less than one metre of soil depth. The key to complete removal is avoiding high application rates (i.e. overloading) and maintaining good site drainage to prevent surface water and groundwater impact on the disposal area. The avoidance of high application rates can be achieved by matching design loads to soil conditions using conservative application rates where groundwater protection for household supply purposes is the objective. Under these conditions regulatory controls setting a 600 mm to 900 mm clearance (the larger value for coarser sand soils) between the base of disposal field and highest groundwater level, with a 20 m horizontal clearance to nearby water supply bores, would appear adequate. Where the 600 mm to 900 mm clearance is not achievable within the natural soil, then imported fill of no less a quality than natural soil can be used to build up the disposal area to achieve the required clearances. In situations where the use of groundwater is not required for water supply purposes there may well be a case for reducing these clearances down to a 300 mm minimum. This ensures that good use can be made of the natural soil for bacterial reduction immediately adjacent to the infiltrative surfaces of the disposal area. Comment: Note that this review deals solely with septic tank effluent land application, and not improved effluent quality from aerobic treatment plants or septic-tank/sand-filter treatment systems. Furthermore, the 20 m clearance to water supply bores is again entirely arbitrary, and does not take into account the factors identified in the Wellington Regional Council viewpoints on such clearances (see 10.0 above). (b) ASAE On-site Symposium Review: On-Site NewZ has reviewed [Ref.11] selected conference papers from the ASAE March 2001 event, summarising latest US research findings. Several papers discussed contamination into groundwater from on-site systems. [Item 16] Noted that septic effluent discharges into sandy soils on 800 sq.m lots at a beach resort community found very low (less than 2 cfu/100 ml) E.coli levels and variable nitrate levels. Beach seep from shallow groundwater contained nitrate levels above drinking water standards, but no faecal indicator organisms. [Item 18] A laboratory and field study investigated bacteria and virus removal in soils. A field test of a mature 8 year old soil absorption system expected to show greater than 3 log removal in 600mm to 900mm of sandy soil, but in fact found that this was achieved at between 5 and 600mm depths. Complimentary laboratory studies showed that non-mature systems are likely to result in periodic breakthrough of bacteria and virus during early operation. A 3 log reduction in virus and near complete removal of bacteria was found within 600mm and 900mm depth in sandy soils. Additional removal of viruses is expected in the saturated zone moving away from the soil absorption area. A further finding was that faecal coliform levels may reflect virus levels in the soil immediately below the infiltration surface of a land application system. [Item 23] This reported an assessment of the impacts of drip irrigation systems on groundwater quality, and confirmed that a clearance to seasonal groundwater of 300 mm was acceptable. This clearance was too low where bedrock or hardpan was present, as extra depth is needed to cope with the hydraulic flow regime in the restrictive shallow soil conditions. Comment: The conference presentations confirmed that system design can be utilised to provide appropriate protection of groundwater quality. (c) ESR Research Study: During a 1996 study into protection zones for water supply springs in the Rotorua area [Ref. 12], Christchurch ESR carried out a laboratory study of the bacterial and viral removal capacity of local soils. This study involved passing doses of septic tank effluent vertically upwards (to ensure saturated conditions) through a 1 metre depth of pumice sand column. The application rate was 0 mm per day. The objective (section 5.3) was to obtain some understanding of the fate and transport of microbes in the groundwater saturated zone. Results showed (section ) that 100% removal of both bacteria and bacteriophages (replicating behavior of viruses) was achieved through the 1 m depth of the sand columns. Filtration was believed to be the dominant removal mechanism. With time, there was a gradual decline in flow rates, indicating slow clogging by fine suspended matter in the applied effluent. The report stated that: On-Site NewZ Setbacks and Clearances (Historical overview) [23 January 2014] Page 6

7 The fact that microbes can be significantly removed through a pumice sand layer means that the disposal of effluent onto pumice soils is practicable. It can be inferred that with continuous input of effluent, particles of suspended solids and bacteria will gradually clog the system and reduce the permeability of the underlying soil. As a result of clogging surface ponding could occur. This is a common phenomenon in the field. Most septic tank ---- (effluent) infiltrates quickly at first and then forms a self-sealed layer after some time of use. Comment: This study confirms the known high treatment capacity of soil systems in removing bacteria and viruses. The conditions under which the laboratory columns achieved complete removal of microbes (i.e. 1 m depth of saturated flow through pumice sand at 0 mm/day) are quite extreme relative to design criteria for, say, on-site soakage trenches. First, trenches in sand are loaded at some ten fold lower loading rates than applied during the studies. Second, clearances from trench bottom areas to groundwater ensure that unsaturated flow conditions are achieved through the soil layer being used for treatment (i.e. bacterial and viral capture and retention). Un-saturated flow conditions improve significantly both retention and die-off rates for such microbes. The observed conditions of slow clogging of the pumice sand under continuous loading showed the development of the natural clogging layer (slime growth) that coats the infiltrative surface over time of all septic effluent soakage systems. Eventually, this clogging layer reaches full development in equilibrium with the anaerobic/aerobic biological decay processes inherent in the soil/effluent microbial actions. At this point on-site practitioners refer to the fact the system has reached its LTAR (long-term-acceptance-rate). LTAR represents the rate at which effluent is absorbed through the biological clogging mat and at which the resulting treated carriage water moves into the surrounding unsaturated soil. This active clogging mat plays a key role in microbial retention and inactivation through dieoff in competition with the aggressive soil microbial regime in the clogging slime layer. Detailed research has shown that the LTAR of a fully developed mature clogging layer has an infiltration rate for carriage water of some 10 mm/day (i.e. around times lower than the study application rates). The LTAR value varies for differing soils dependant on the penetration of bacterial slimes in depth into the pores of the soil. Hence porous soils have higher LTAR values than less porous. Design loading rates for on-site subsoil soakage systems are always set below LTAR values so as to avoid ponding and breakout to ground surface of applied effluent. Septic tank flows (primary effluent) will create denser clogging layers than aerated treatment plant or sand filter flows (secondary effluent). Indeed, research in the US shows that treating septic tank effluent to secondary quality will increase the LTAR capacity of soakage trenches by a 10 to 12 fold factor (the reason being that clogging slime build up is significantly reduced). However, design loading rates for trench systems to which secondary effluent is allowed under AS/NZS 1547:2000 are only twice that indicated for septic tank effluent. The reason is that a clogging layer is still desirable in aiding bacterial retention and dieoff, and higher loading rates would only push microorganisms further through the soil and carry them into groundwater. (d) The NIWA Study for Rotorua Lakes: This study [Ref.13] was undertaken for the Rotorua District Council to assess the potential for septic tank leachate to contaminate lake waters where urban areas utilising septic tank systems are adjacent to lake margins. The main contaminants of concern are pathogens and nutrients (mainly nitrates). The Council also asked NIWA to look at groundwater horizontal flow rates toward the lake from a potential communal soakage trench some distance away from a strip development of existing lots. This indicated a flow time of 22 months per 100 metres. However, NIWA considered vertical separation distance to be the more significant control over bacterial contamination than use of horizontal clearances, and noted that the regional council (Environment BOP) has already set a 600 mm minimum within the Regional Plan, and requires special distribution methods for effluent into coarse soils. The report also makes the following general conclusions: The use of horizontal separation distances to control development at the lake margin areas is in many cases not practical. In any event, setting vertical separation distances is preferable. This enables use of the treatment capacity of un-saturated soils to achieve very effective removal of pathogens. Design, installation and monitoring of septic tank soakage fields should ensure that soils below the field system down to highest groundwater do not become saturated Findings Vertical clearances: Appropriate vertical clearances can be set for on-site wastewater effluent application to soils so as to prevent bacterial and viral contamination of the groundwater. These distances can vary according to the type of soil, the quality of effluent (as determined by the pretreatment level and process), the loading rate for the effluent to the soil infiltrative surface, the application regime (whether gravity trickle loaded or pressure dose loaded), and the likely use of the groundwater resource. Conventional gravity loaded On-Site NewZ Setbacks and Clearances (Historical overview) [23 January 2014] Page 7

8 trench systems for septic tank effluent are usually designed and constructed to ensure a minimum clearance to seasonal watertable level of 600 mm. Alternative design approaches can be utilised to handle situations where groundwater levels are closer to the ground surface under wet seasonal conditions. Horizontal clearances: A wide range of such clearances is utilised in practice. All of them are quite arbitrary, and appear to be based around the values presented by the US Public Health Service back in 1957 (see 1.0 above). They effectively provide a factor of safety in design and location of on-site systems in furtherance of maintaining environmental and public health protection objectives. The US PHS qualified such values as guidelines to be used in the absence of detailed site information. However, many agencies in the past have adopted then as almost inviolable requirements, without any flexibility allowed in their application. This seems to be based on a presumption that they have been developed around a sound scientific foundation, and that straying from them would be a breach of that foundation. There has also been an attraction for regulatory authorities to have quite prescriptive requirements that avoid the need for competence in assessing alternative designs aimed at addressing actual environmental conditions in specific circumstances. This creates a problem alluded to by Wellington Regional Council (see 10.0 above). The Council guidelines point out that stipulating a separation distance from bores may provide a false sense of security to people drinking water from the bore when in fact contamination from upslope may be travelling to the bore from well outside the clearance area. In summary: Separation distances to groundwater and surface waters have in the past been set to provide arbitrary factors of safety based upon use of septic tank and soakage trenches. There is no consistency in the values adopted. They probably reflect an expectation that such on-site systems are not a fully reliable means of wastewater servicing, and that poor performance can be compensated for by an appropriate buffer between the system and water sources used for community and/or individual household supply. Research and practice experience is now available to confirm that properly designed on-site systems can totally retain microbiological contaminants within the land application system and thus protect natural waters both on-site and off-site for subsequent use. Design methods enable variable vertical separation distances to be utilised. Arbitrary horizontal distances are still set by some Regional Councils as factors of safety for use in District Council approvals. However, Regional Councils are able to consider reduced clearances on a case by case basis through their consents procedures. REFERENCES ******************** 1. USPHS (1958) Manual of Septic Tank Practice, US Department of Health Education and Welfare, USPHS Publication No. 526, Bureau of State Services, Division of Sanitary Engineering Services. 2. USPHS (1967) Manual of Septic Tank Practice, US Department of Health Education and Welfare, USPHS Publication No. 526, Bureau of Disease Prevention and Environmental Control, National Centre for Urban and Industrial Health Cincinnati, Ohio NZ Standards Institute (1961) Disposal of Effluent from Household Septic Tanks, Code of Recommended Practice, CP 44:1961, NZ Standards Institute, Wellington. 4. US Environmental Protection Agency (1980) Design Manual - Onsite Wastewater Treatment and Disposal Systems, Office of Research and Development, Municipal Environmental Research Laboratory, Cincinnati OH New Zealand Standards Council (1982) Household Septic Tank Systems, NZS 4610:1982, Standards Association of NZ. 6. Gunn, Ian (1994) On-site Wastewater Disposal from Households and Institutions, ARC Environment Technical Publication No. 58, Auckland Regional Council, Second Edition, November Standards Australia, Standards New Zealand (2000) On-site Domestic Wastewater Management, AS/NZS 1547: horizons.mw (2000) On-site Wastewater System Guidelines for the Manawatu-Wanganui Region, Report No. 20/EXT/381, horizons.mw, Palmerston North, November Wellington Regional Council (2000) Guidelines for on-site sewage systems in the Wellington Region, WRC/RP-G-00/47, Wellington Regional Council, Wellington, December Gunn, Ian (1997) On-site Wastewater Systems and Bacterial Reduction in Sub-soil Disposal Areas A Review, On-Site NewZ Special Report 97/2, Department of Civil and Resource Engineering (CaRE),The University of Auckland, 5 May Gunn, Ian (2001) On-site Wastewater Treatment A Report on Selected Papers from the ASAE Ninth Symposium on Individual and Small Community Sewage Systems, Fort Worth Texas, March 11-14, 2001, On-Site NewZ Special Report 01/1, A CaRE for the Environment Project, April Pang, Liping, Close, Murray and Sinton, Lester (1996) Protection Zones of the Major Water Supply Springs in the Rotorua District, Report No. CSC 96/7, Christchurch Science Centre, Institute of Environmental Science and Research. 13. 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