Chris Stapleton introduces a proposed new methodology for tracking the effects of development on soil
Soil functions are important components of terrestrial ecosystems, and in my Transform article (November 2019), I set out the most important land and soil inputs for environmental impact assessments (EIAs), as we move towards a terrestrial ecosystem services approach to these resources.
If the focus of protecting land and soil is beginning to encompass the provision of a wider range of terrestrial ecosystem services than the production of biomass (food, fibres etc) from agricultural land, how can this be done within EIAs? Here I set out one possible approach with reference to the content of environmental statements (ES) which report on and summarise the EIA process.
Impacts and mitigation
The most important impacts of development are the land take in terms of the land use changes (not just from agriculture to urban) and the displacement of soils, both permanently and temporarily. For impact avoidance, site boundaries and the design of schemes can be adjusted to ensure that the smallest area of land is lost or disturbed. Layouts can be configured to locate hard development on less valued land and soils, and to maintain the physical viability of residual agricultural land.
As for mitigation, soils displaced (both permanently and temporarily) should be quantified and conserved for sustainable residual end uses. For temporarily displaced soils this can be achieved by putting them back where they came from, and for agricultural restoration the reinstatement of land to its original quality is the main mitigation objective.
Finding a suitable location (preferably within the red line of a development site for effective development control) and the sustainable reuse of permanently displaced soils, to retain their natural functions, is a greater challenge. This often requires larger sites to facilitate handling of the greater soil volumes generated, but this does not necessarily increase the hard development footprint. However, site boundaries have to be adjusted at an early stage of a development project.
Soils are an environmentally valuable natural resource and a special category of 'excavated material'. Therefore, good project design and well-managed construction operations must ensure the sensitive handling of topsoils and subsoils separately from the bulk movement of other excavated materials.
Changes to EIAs and ES
ES generally have a section or chapter on Agriculture, setting out the areas of Agricultural Land Classification grades/subgrades of land taken by the proposed development. However, soil handling proposals for the conservation and sustainable use of displaced soils are often overlooked, even when large volumes of soils are potentially lost.
The shift in focus to a terrestrial ecosystems approach for the protection of a wider range of soil functions than the production of food, fibres and timber should now be reflected in a more generic ES section or chapter simply entitled Land and Soils. This would help us to concentrate on the most important impacts of development on land and soils and the mitigation measures set out above. It would recognise the significance of land take and soil displacement in ecosystem services terms, particularly in respect of land use changes from agricultural land to different categories of hard and soft development.
A Land and Soil section or chapter would set out the areas of agricultural land (in hectares) transferred to different types of hard and soft development within a scheme, together with an account of what has been done with the soils displaced, both permanently and temporarily. There would also be a commentary on the maintenance of soil functions for the provision of terrestrial ecosystem services. For example, opportunities for the use of permanently displaced soils to establish groundcover on brownfield land intentionally included within a proposed development site could be more clearly presented and understood as a sustainability gain arising from a proposed development.
A worked example
Residential development often involves a change from agricultural land to hard development, with some largely undisturbed soils in residual green spaces alongside roads and between the houses (ie soft development areas). Currently, there is no requirement and few opportunities within the planning system to find a sustainable use for soils displaced by hard development. Finding off-site uses has implications for transport costs and other impacts, but we must question where these valuable and finite natural resources are currently going, as they seem to disappear without trace. There are, however, opportunities for the sustainable use of displaced soils that would otherwise be 'lost'.
Table 1 shows how land use changes arising from a proposed residential development can be addressed from a soil functions perspective. The significance of these changes should be assessed with reference to the need to maintain soil functions for the provision of terrestrial ecosystem services. Soil functions are set out in the boxed text. In this relatively simple example, the proposed 48ha development site comprises 40ha of agricultural land with undisturbed natural soils and 8ha of brownfield land with no soil materials present.
Table 1: EIA of land use changes arising from proposed residential development.
Maintenance of soil functions for the provision of terrestrial ecosystem services
* 1 eg Natural undisturbed soils; disturbed natural soils; topsoil or subsoil only; soil-forming materials; no soils; contaminated/uncontaminated etc
* 2 Retained on land controlled by the developer
* 3 Following development land must remain accessible from agricultural land/farms
* 4 Following development land must remain accessible for management
* 5 Following development land must remain accessible to be a source of materials
* 6 Following development land must remain accessible for the recording and preservation of archaeology and cultural heritage features
The proposed development is 40ha of new houses (including some soft development areas), together with 4ha of broadleaved community woodland with public access and 4ha of acid grassland with restricted public access, on the land that is currently brownfield. Both woodland and grassland would be accessible for management by a local trust that would have ownership of these areas. If soil survey has identified a 25cm-deep topsoil with a subsoil extending to a depth of 1.2m (ie the subsoil is 95cm thick) then, using this information, the 40ha of agricultural land would yield approximately 100,000m3 of topsoil and 380,000m3 of subsoil, minus the volume (not known for the purpose of this example) of soils retained in situ in the residential soft development areas.
Both topsoil and subsoil could be used for the establishment of the woodland, but a suitable low nutrient substrate of subsoil only would be required for the acid grassland. A soil profile with 25cm of topsoil and 95cm of subsoil over the 4ha of woodland would use 10,000m3 of the displaced topsoil, and 38,000m3 of the subsoil. A soil profile of subsoil to a depth of 1.2m over the 4ha of acid grassland would use a further 48,000m3 of the subsoil.
According to this audit of soil resources, 90,000m3 of topsoil and 294,000m3 of subsoil would have to be relocated off-site or otherwise lost. Alternatively, more of the subsoil could be placed over the 8ha brownfield land to a greater thickness, with some topographic landscaping where appropriate.
Clearly, there are limits to what can be achieved in terms of soil conservation, given the lack of opportunity within the planning system to find a sustainable off-site use for displaced soils, but this approach would quantify the scale of the loss of this finite resource.
In addition to this audit of land use changes and volumes of displaced soils, this part of the ES would have a narrative describing the way in which soil functions would be maintained or lost. For instance (and in simple headline terms for the purpose of this exercise), the proposed development would have the following effects:
Biomass production
There would be a loss of biomass production from the 40ha of agricultural land, and this can be quantified.
Ecological habitat
In this example, the current value of the brownfield habitat is not known, but there would be the loss of 40ha of relatively low value habitat from the agricultural land. A consideration of soil functions would form part of a biodiversity net gain assessment carried out on the land use and habitat changes.
Community woodland
There would be biodiversity gains (possibly in terms of species diversity) and recreational gains from the 4ha of community woodland with public access.
Acid grassland
There would be biodiversity gains (possibly in terms of species diversity) from the 4ha of acid grassland.
Component of carbon cycle
The topsoil of the agricultural land is a carbon store, and the conservation of the displaced soil for a sustainable use would maintain this function. The biomass production harvested annually from the 40ha of agricultural land extracts CO2 from the atmosphere, and this would be replaced to some extent by 8ha of permanent woodland and grass cover. The balance can be quantified.
Component of hydrological cycle
The use of fertilisers and other chemicals for agricultural production would cease. Subject to the installation of effective sustainable urban drainage systems and pollution control measures for the residential development, infiltration rates should be maintained and water quality improved within watercourses and groundwater. The introduction of soil and a groundcover on the brownfield land would enhance the contribution of this land to the hydrological cycle.
Source of materials
Worked out surface mineral sites present a blank canvas for the reinstatement of soils, together with the introduction of soil functions and a wide range of sustainable end uses. Unless the proposed development was preceded by extraction of any materials present, they would be sterilised under the hard development.
Archaeology and cultural heritage
Features of archaeological or cultural value within the hard development area would be recorded and preserved.
Proposed methodology
Environmental specialists within EIA teams would contribute towards this analysis and commentary to determine the balance of sustainability achieved with respect to soil functions and land use changes. The methodology combines a degree of quantification commensurate with our understanding of the various EIA topics, together with objective analysis by specialists. In particular, and for the biodiversity soil function, there would be scope for ecologists to determine any biodiversity net gain and offsetting.
Similar tables can be produced for other types of development proposals with a different mix of land use changes, and where there is scope for the sustainable use of displaced soils.
This approach to topics within the EIA process represents a useful framework with a comprehensive checklist of considerations and it provides a better understanding of the effects of development on land and soils than current practice. The impacts on agriculture and farm holdings can be more appropriately addressed in existing ES sections and chapters on social and economic impacts.
Chris Stapleton is a member of the Royal Town Planning Institute and IEMA.
