Land Capability Assessment (Science and Compliance) EHO Version

Transcription

1 Land Capability Assessment (Science and Compliance) EHO Version Presented by Paul Williams

2 Introduction 1 This presentation has evolved from a series of presentations for Goulburn-Murray Water. The previous presentations were mainly directed at LCA practitioners (particularly recalcitrant practitioners). The principles, guidance and advice in the original presentations remain essentially unchanged but have been revised in the context of use by EHOs to assist in the assessment of LCA reports. You will see some things repeated a number of times. This is intentional.

3 Introduction 2 In the State of Victoria, land capability assessment needs to comply with: SEPP Code of Practice Onsite Wastewater Management AS/NZS1547 MAV Domestic Wastewater Management Plans Memoranda of Understanding Note: These documents chiefly set compliance boundaries. They do not tell you how to undertake or assess a LCA.

4 Introduction 3 Codes and Standards are created from policy, vested interests, political science and sometimes some real science. An over-bearing sense of low expectation has greatly influenced our current Codes and Standards. They follow a generalist and conservative approach. Generalist approaches generally work for general conditions. One does not often encounter general conditions in the field. e.g. conservative designs are achieved for loam soils (the general soil), but what happens at the extremes, particularly, say, Type 5 and 6 soils? For these soils (which arguably are the general soils in Victoria), the design guidelines in the Code and Standard do not work. The inbuilt intended conservatism can actually promote system failure.

5 Introduction Consider Code Table 9: Type 6 Soils (medium to heavy clays) are predominantly allocated an indicative permeability of <0.06m/day. What does <0.06m/day mean? How much less than 0.06m/day? What if it is near or at zero m/day? Example: Shepparton Formation clays at a Cobram site are not high swelling, not sodic and not dispersive but have measured (in situ or laboratory determined) hydraulic conductivity in the range of m/day to 0.01m/day. The smaller value is not far from being considered impermeable according to the EPA. This illustrates how a general approach can break down.

6 Land capability assessment stripped down Section of the Code describes a 12-stage process as a best practice procedure for land capability assessment. This presentation focuses on the soil profile characteristics (and in particular Type 5 and 6 soils). A critical element of the land capability process is to adequately characterize the soil profile. If this characterization is not done satisfactorily, the LCA is worthless. There are 3 critical elements:

7 The trinity 1. Profile thickness (including a topsoil interval). 2. Profile hydraulic properties (incl. colloid stability). 3. Nutrient uptake and pathogen attenuation ability. The LCA must characterize these 3 aspects.

8 How much detail? The assessor needs to acquire and demonstrate sufficient information to enable the rational design of an on-site system.

9 Soil Description and Profile Thickness Adequate (design) renovation of the effluent requires a minimum thickness of suitable soil. For septic trenches, the Code requires a minimum thickness of suitable soil between the base of the trench and a limiting layer of 0.6m. For a typical 0.6m deep trench, the minimum soil depth is 1.2m. If we consider the possibility of groundwater mounding on an impermeable layer, this minimum design soil thickness becomes about 1.4m. Similarly, for subsurface irrigation lines, the minimum design soil thickness is about 0.9m. Classic limiting constraint.

10 Soil Description and Profile Thickness (cont d) What about soil description? While it is important to be able to communicate the soil description via a uniform or universal code it requires significant experience and training to be accurate and consistent. Soil classifications in common use within the land capability assessment industry include: Unified Soil Classification System (USCS) published by the American Society for Testing and Materials (1985) and variants used by geotechnical practitioners and used by AS2870 and related standards;

11 Soil Description and Profile Thickness (cont d) USDA (Casagrande) Soil Survey Manual (1951) from which the soil textural classification and other terminology is used in the Code and in AS/NZS 1547:2012; The Northcote classification (1979) based on the soil profile form, the overall visual impact of the physical soil properties in their intimate association with one another within the framework of the solum. This system is based on observable physical, chemical and biological features and properties, and not on the mode of soil formation (soil genesis); Handbook of Australian Soils (1968) which distinguishes Great Soil Groups based on soil properties and features related to the processes of soil formation;

12 Soil Description and Profile Thickness (cont d) The Australian Soil Classification (1996), which is a general purpose scheme based on defined diagnostic attributes, horizons, or materials, which are largely observable in the field and is not too reliant on laboratory data; and, Hybrid classifications (which range from quite practical to downright irrational). All systems, however, describe the soil profile in terms of horizons, i.e. variations (usually textural) with depth. End users (and many assessors) and administrators can, not surprisingly, be greatly confused. This can and has led to irrational decisions causing an unnecessary financial burden on an applicant and/or increased risk to public health and water resources.

13 Soil Description and Profile Thickness (cont d) A solution to this quandary is to consider the salient soil and associated land characteristics relevant to trench disposal of septic effluent and irrigation of secondary effluent, rather than describing soil taxonomy aspects to great detail. Regardless of which classification is used, the salient soil characteristics relevant to effluent disposal capability are: Thickness of the profile (including presence of a topsoil horizon), Profile hydraulic properties (including colloid stability), and, Nutrient uptake and pathogen attenuation ability.

14 Soil Description and Profile Thickness (cont d)

15 Soil Description and Profile Thickness (cont d) A topsoil (A 1 -horizon) layer of grey-brown, dry to moist, medium dense sandy silt (silt loam), with a soil reaction trend of 5.5 to 5.6 ph and electrical conductivity of 0.32 to 0.34 ds/m, containing a root mat and root zone, to a depth of 0.1m, overlying, A topsoil (A 2 -horizon) layer of grey-brown, moist, medium dense clayey-sandy silt (clay loam) and clay content increasing with depth, with a soil reaction trend of 5.7 to 5.9 ph and electrical conductivity of 0.21 to 0.29 ds/m, to depths of 0.35 to 0.4m, overlying, A residual soil (B 1 -horizon) layer of yellow-brown, moist, very stiff and moderately well-structured silty clay of low plasticity (light clay), with a soil reaction trend of 5.8 to 6.0 ph, electrical conductivity of 0.29 ds/m and free swell of 30%, to depths of 0.45 to 0.6m, overlying, A residual soil (B 21 -horizon) layer of orange-brown, moist, very stiff and moderately well-structured silty clay of low plasticity (light clay), with a soil reaction trend of 5.9 to 6.0 ph, electrical conductivity of 0.29 to 0.34 ds/m and free swell of 30% to 35%, to depths of 1.3 to 1.6m, overlying, An extremely weathered (B 22 -horizon) layer of orange-brown and red-brown, moist, very stiff and moderately well-structured sandy-silty clay of low plasticity (light clay), with a soil reaction trend of 5.9 ph, electrical conductivity of 0.32 to 0.36 ds/m and free swell of 35% to 40%, to depths of 1.85 to 1.95m, overlying, Highly and less weathered sedimentary rock.

16 Profile hydraulic properties The Darcy equation states that velocity of a liquid through a porous medium is the product of the hydraulic conductivity and the hydraulic gradient. Hence, knowing the hydraulic conductivity allows the estimation of deep seepage and flow times which allows confident disposal system design and demonstrates the adequacy of buffer distances to sensitive areas and entities. Hydraulic conductivity of many soils can easily be measured insitu. AS/NZS 1547 directs the practitioner to determine an indicative permeability by assessing the soil s texture (proportions of silt, sand and clay) and structure. This makes some theoretical sense but is a practical nonsense.

17 Profile hydraulic properties (cont d) To demonstrate the texture method flaw, consider the category 1 soils (gravels and sands) which the Code and Standard assigns an indicative permeability of more than 3m/day and allows less than 5% clay content. Under certain moisture and compaction conditions, such a material could make a superb base course for a road pavement and would be considered effectively waterproof with a permeability of less than 0.001m/day. Similarly, soil structure assessed by visual inspection of pit walls and exposures may not be the same structure when the soil is cyclically exposed to saline effluent.

18 Nutrient uptake & pathogen attenuation (cont d) 1 Hydraulic conductivity is easily measured in situ and when coupled with some (simple) laboratory testing to determine colloid stability (dispersion and swell potential) provides a high degree of certainty in hydraulic design. Several processes affect nitrogen levels within soil after application of effluent. Alternate periods of wetting and drying with the presence of organic matter promotes reduction to nitrogen gas (denitrification). Plant roots absorb nitrates at varying rates depending on the plant species, however nitrate is highly mobile, readily leached, and can enter groundwater via deep seepage.

19 Nutrient uptake & pathogen attenuation (cont d) 2 To ensure complete attenuation of nitrogen, a nitrogen balance is used with conservative estimates of the nitrogen uptake by different plants. Sufficient LAA area should be used to encourage wetting/drying cycles within the effluent field to stimulate microbial attenuation of nitrogen. Clay subsoils fix large amounts of phosphorous and a phosphorous balance should not be required. A small amount of nitrogen, as nitrate, will inevitably reach the groundwater. However, this nitrogen from the effluent would be insignificant in the context of the nitrogen routinely applied in common farming practices in the vicinity.

20 Nutrient uptake & pathogen attenuation (cont d) 3 Furthermore, the time taken for the effluent to reach surface waters (a minimum distance of, say, 40m) and assuming a prevailing hydraulic gradient of 1:500 and ksat of 1m/day, would be in the order of 50 years. For rare perched water flow in the topsoil materials (subsurface storm flow) the time taken for the effluent to reach surface waters (a minimum distance of, say, 40m) and assuming a prevailing hydraulic gradient (ground slope) of 1:10 and ksat of 0.5m/day, would be in the order of 2 years and assumes no evapotranspiration during this time.

21 Nutrient uptake & pathogen attenuation (cont d) 4 Reduction of faecal bacteria with distance of travel

22 Permeability the Code however clause In situ constant head permeability testing is considered best practice in the Code. While it allows the assessor to use the visual/tactile ( spit ball ) method or constant head method, it is clearly stated that However, should there be a dispute or any doubt or uncertainty regarding the soil category derived by visual/tactile methods, in situ permeability testing must be undertaken. The Code also states that the permeability testing is best practice and is to be undertaken in the limiting layer.

23 Permeability testing the limiting layer In situ constant head permeability testing must be representative of the limiting soil layer. In Victoria, this limiting layer will mostly be a clay subsoil (Type 5 and Type 6 soils). Numerous presented LCAs have contained irrigation area sizing based on an indicative permeability of the topsoil. One consultant averages the indicative permeabilities of topsoil and subsoil in proportion to the layer thickness what utter nonsense!

24 Type 6 Soils assessing permeability Type 6 Soils: Medium clays, heavy clays, swelling clays, sodic soils, magnesic soils, dispersive soils, hardpan? In situ permeability testing is not rational for these extreme Type 6 soils (see Code 3.6.1). A design permeability can be derived using laboratory determined soil ameliorant quantities (based on Cation balance), simple swell tests and precedent. Note again the Code restriction on DIR for Type 6 Soils.

25 Type 6 Soils soil amelioration For swelling, sodic, magnesic and dispersive soils, gypsum is commonly used to create and/or maintain water-stable peds thus creating/maintaining an appropriate design hydraulic conductivity. The quantity of gypsum or other ameliorant necessary to achieve a suitable soil condition (balanced cations) can vary significantly: Many land application areas require less than 1kg/m 2 gypsum, however, the gypsum requirement can be much larger, e.g. sites in Central Goldfields, Moyne and Strathbogie (sodic, magnesic, swelling and dispersive clays) can need up to 35 tons/ha gypsum to be able to use a DIR of 2mm (before allowing for rainfall).

26 Common Errors and some Strange Statements 1 As EHOs, when you encounter any of the following in an LCA you need to very critical of its recommendations: Designing for the topsoil. Not considering nutrient uptake requirements. Ignoring key laboratory results (e.g. swell potential). Statements like: We are only interested in the top 800mm of soil. Recommending an irrigation design on a 38% slope without allowing for the slope or determining profile thickness. we recommend that the testing be conducted when the permeability improves.??????? Averaging indicative permeability for different soil horizons.

27 Common Errors and some Strange Statements 2 The most important consideration is the reactivity of the clays. That was the last mention of or allowance for the reactive clays. Presenting 2 identical data sets for 2 separate but nearby sites tested one month apart (the same data sets gave different ksat values). Using AS/NZS1547 Tables instead of the Code Table 9. Failure to assess clay potency. Hydraulic testing over multiple texture horizons. Constant head testing after 24 hours soaking, or when the soil is saturated or in swelling clays (and combinations). Test data not presented in reports. Ignoring (or not understanding) limiting constraints.

28 Common Errors and some Strange Statements 3 Changing the soil type description from light clay to clay loam (with resultant higher indicative permeability) to enable the use of a restricted site. Accepting pressure compensated subsurface irrigation lines in a trench with gravel. This is neither a valid trench design nor a valid irrigation design it certainly is not pressure compensated! Accepting pressure compensated subsurface irrigation lines not level/parallel with contours. Constant head testing after 24 hours soaking, or when the soil is saturated or in swelling clays (and combinations). Test data not presented in reports. Ignoring (or not understanding) limiting constraints. Remember, always apply the pub test!

29 Case study (a quick review of a recent LCA) 1 Soil surface condition: Soil was wet and water was observed lying on the surface during testing. Soil type: highly plastic clay.. A constant head test showed that the percolation rate was very low Hence, recommend a secondary system.permeameter testing in wet soils, swelling soils? Bore hole to 1.1m. Code requires 2m. Soil category and permeability: A constant head test was conducted to determine the indicative permeability in accordance with AS 1547 Appendix G. Test was not done in accordance with Appendix G. Insitu testing is to measure the actual permeability where does indicative permeability in AS1547 come in? Details test and results in wet, swelling clay (???).. concludes it is a Category 6 Soil with permeability m/day!

30 Case study (a quick review of a recent LCA) 2 Proposes to use secondary effluent into evapotranspiration trenches.???? In accordance with AS1547 Table L1 the soil category for highly expansive clay is category 6 and therefore the recommended design loading rate for this system is 5mm/day. Does not follow table L1 notes (no special design) water balance is for irrigation using rainfall with less than 30 years record. incorrect trench spacing No mention of soil ameliorants etc. Use of AS1547 instead of Code Table 9. 5mm/day is the maximum loading rate. The recommended system design is derived from irrational testing, misinterpretation of the Code and irrelevant test and modelling methods.

31 Case study (a quick review of a recent LCA) 3 Note: A rational design for this site is 565m 2 LAA, DIR 1.3mm/day, 2.5kg/m 2 gypsum (from laboratory testing), deep ripping and addition of topsoil (from house and driveway stripping). Talk to your Wyndham peers regarding the extraordinarily large dimensions required for trenches and beds in swelling clays.

32 A Recap of Key Points 1 SEPP (Waters of Victoria) require that any proposal be assessed on a risk-weighted basis. The LCA must present sufficient information to enable the rational design of an on-site system (in accordance with the Code). The receiving soil must be characterized in terms of: 1. Profile thickness. 2. Profile hydraulic properties (incl. colloid stability). 3. Nutrient uptake and pathogen attenuation ability. The testing and design must include the limiting layer. You must characterize and design for all limiting constraints (profile, climate, topography, hydrogeology, hydrology).

33 A Recap of Key Points 2 Note: Not all EPA-approved treatment system types are applicable for every site. It is important that the most appropriate type is selected to suit the characteristics of the site, the climatic conditions, the needs (Code 1.9) The testing and design must include the limiting layer. You must characterize and design for all limiting constraints (profile, climate, topography, hydrogeology, hydrology). Remember, if you don t understand elements of the LCA, chances are that the writer doesn t either.

34 Water and Nutrient Modelling in High Rainfall Regions (a question of balance) Presented by Paul Williams

35 Water balance General water balance analysis for subsurface irrigation: R + Eff + SF + SubF = Et + RO + IF + DI + ΔS Where R is incident rainfall, Eff is applied effluent, SF is surface flow (run-on), SubF is interflow from upslope (perched groundwater), Et is evapotranspiration, RO is surface flow from the LAA (can only be excess rain water), IF is interflow, DI is deep seepage, and, ΔS is soil moisture change.

36 Water balance By installing a cut-off drain, SF and SubF are eliminated. For slopes less than 10% IF can be ignored. In the long-term and considering soil hydraulics only, ΔS can be ignored. Hence, the equation reduces to: R + Eff = Et + RO + DI Where R is incident rainfall, Eff is applied effluent, Et is evapotranspiration, RO is runoff (can only be excess rain water), DI is deep infiltration (deep seepage).

37 Water balance Domestic, high rainfall 1

38 Water balance High rainfall, no effluent 2

39 Water balance - discussion What is the fate of the retained rainfall? It is predominantly deep seepage (see WB 2). For our test site and for zero applied effluent and to limit surface flows to about 10%, a deep seepage of 6.6mm is required..

40 Water balance - discussion Design suggestions for high rainfall areas: Always measure ksat. Minimise water use. Maximise treatment. Maximise the LAA to a practical/sensible size. Is the balanced LAA in WB 2 practical or sensible? It is if you can reduce water usage.

41 Water balance - discussion Design suggestions for low to medium rainfall areas: Always measure ksat. Select the larger of Hydraulic LAA and nitrogen uptake LAA. Note: For grasses and Type 2 to 5 soils DIR can t be larger than 2.5mm/day. Does this mean the Code Table 9 DIRs larger than 2.5mm/day are wrong/irrelevant?

42 Water balance 9 th decile wet year rainfall For open potable water supply catchments and other sensitive areas design should include consideration of 9 th decile wet year rainfall. Note: This is not 9 th decile monthly rainfall. Use of consecutive 9 th decile monthly rainfall would mean designing for a 1 in 1 trillion event.

43 9 th decile wet-year rainfall

44 System failure (1) Some onsite (irrigation) systems have failed with resultant surface expression of effluent. What caused them to fail? If we consider only soil hydraulics and the inherent in-built factor of safety (deep seepage less than 12% Ksat for deep profiles) and given that it is possible to accept a transient loading of up to 99.9 % ksat without surface expression of effluent, it is difficult to explain a failure due to (even severely) increased hydraulic loading. Apart from construction, maintenance and operation defects or damage (ploughed LAA, pugged LAA, outlet pipe disconnected from the treatment plant etc), failure can often be traced to grossly underestimated effective soil hydraulic properties.

45 System failure (2) In situ measurement often reveals an order of magnitude range in Ksat, while 2 orders of magnitude range is not unknown (but would certainly generate further detailed investigation prior to design). It would not be surprising if an assessor s guess at Ksat (from the Code, Table 9) was at least an order of magnitude wrong. If we consider a certain Type 5 soil (Ksat <0.06m/day) and the assessor s guess is 1 order of magnitude wrong, at best Ksat could be 0.006m/day. At the recommended maximum DIR of 3mm/day (Code, Table 9), the factor of safety is only 2.

46 System failure (3) Assuming reasonable standards in system construction, maintenance and operation a moderate transient increase in load is unlikely to cause surface expression of effluent. Furthermore, if dispersion and/or the profile thickness were not adequately established, system failure would not surprise. If we consider a certain Type 5 soil (Ksat <0.06m/day) and the assessor s guess is 1 order of magnitude wrong, at best Ksat could be 0.006m/day. At the recommended maximum DIR of 3mm/day (Code, Table 9), the factor of safety is only 2. However, assuming reasonable standards in system construction, maintenance and operation a moderate transient increase in load is still unlikely to cause surface expression of effluent.

47 System failure (4) Now, if we add any combination of the following: Undiagnosed dispersion (ksat could be close to zero), severely constrained profile thickness (subsurface mounding), sloping irrigation lines and/or irrigation lines in gravel-filled trenches, system failure would not surprise.

48 System failure (5) The LCA assessment must establish: Thickness of the profile (including presence of a topsoil horizon), Profile hydraulic properties (including colloid stability), and, Nutrient uptake and pathogen attenuation ability. The LCA assessment must recommend rational design elements.

49 Recommended reading Wastewater Subsurface Drip Distribution Peer Reviewed Guidelines for Design, Operation and Maintenance Tennessee Valley Authority, March 2004 Mansfield Shire Domestic Wastewater Management Plan Pilot Project Part 2 Domestic Wastewater Management Plan Attachment 1