Quarrying and Soils – A case for updating

First published in the May 2024 issue of Quarry Management

A case for updating the scope of soil assessments and guidance for mineral workings, by Dr Neil Humphries FRBS FBSSS FIQ

Like minerals, water, the atmosphere, and biodiversity, soils are essential natural capital resources needed for the sustainability of modern society[1]. Soils are also a major determinant of biodiversity[2,3], whilst also contributing to water and air quality. Yet, soils are treated with less seriousness when it comes to considering their protection, conservation, and reuse in quarrying and other developments.

Partly, this has been because of the recent overshadowing by the degradation of soils by agricultural practices[4], but also due to the lag in the updating of planning guidance, which is some 30 years behind the times. It still focuses, important as it is, on agricultural land quality[5,6,7,8]. Unless the land is in agricultural use and is assessed as best and most versatile (ALC Grades 1, 2 and 3a), there is seemingly little interest in mineral schemes[9].

There needs to be a sea-change in the attitude to soil conservation and reuse for the simple reason that soils are, in the long term, a finite resource providing essential ecosystem services. Besides, they will be key to achieving the Government’s aspiration for biodiversity recovery and the requirements in England of the new Biodiversity Net Gain (BNG) regulation[10].

The aim of this paper is to highlight three matters that urgently need to be addressed.

Updating soil assessments

The first matter is the need for a major overhaul of the scope of soil-related considerations at the planning assessment, design, and implementation stages.

Despite the recognition that soils are natural capital and have wider implications for ecosystem services and biodiversity recovery, pre- and post-development soil resource assessments have continued to be based upon their agricultural land use potential[11]. This is set out in the Agricultural Land Classification for England and Wales[5] (also applicable to Northern Ireland), and the Land Capability for Scotland[12]. Whereas the same soil characteristics, landform, and climate conditions have relevance to soil-based ecosystem services that influence flooding, water storage[13], and biodiversity[2,3], it is the author’s contention that soil assessments should now routinely include these.

For example, in the context of biodiversity potential, the suitability of soils, climate and landform contexts for woodlands are set out in Forestry Commission Bulletin 112: ‘Creating new native woodlands’[14]. The warmer lowlands of southern and eastern Britain with brown calcareous/basic earths and base-rich groundwater gleys on various landforms support ash-oak-elm woodland with dog’s mercury, whereas in Scotland, Scots pine woodland is associated with cold and wet upland plateaux/terraces with podzol/podzolic soils. The same can be discerned for other habitats such as grassland[15], heathland[16], and wetlands[17]. The characterization and listing of habitats will need to be converted into the UKHabs typology[18,19] for use in the mandatory BNG assessments.

Where post-development sites involve the quarried landforms and processing wastes, these materials and contexts also have soil-based characteristics[20,21,22,23]. These can be thin, stony and developing skeletal soils[24,25]. In addition, quarried landforms (screes, faces etc) are recognized as habitats of high biodiversity distinctiveness (ie value) in the BNG assessments[10]. Here, for example, lowland oak-birch woodland on sedimentary sands and gravel and juniper woodland on cold and wet upland sedimentary and igneous rock[14] could be appropriate. The same exercise can be done for the other habitats and ecosystem services provided by the soils[26,27].

Fig. 1. Typical agricultural-biased presentation of current soil-assessment surveys

Hence, the pre- and post-development soil and land-use reports should routinely list the habitat potentials. There can be no case for not doing so as the same survey methodology and assessment criteria used in the agricultural assessment, can be adapted[28]. In this way there is no need to imply soils have no value as currently labelled in the reports as simply ‘non-agricultural land’ (fig.1).

Soil conservation and reuse

The second matter is the avoidable loss of the in-situ soil resources. This may not always be possible as the geological context and form of mineral extraction have a fundamental bearing on the possibilities for the conservation and reuse of soils.

There are three basic situations that influence the potential for the reuse of soils: first, mineral deposits with a relatively thick overburden layer that provide sufficient backfill material to create a land surface after extraction and where the soils can be replaced, here the recovered soils are reused (fig. 2a); secondly, those where the workings are above the water-table rebound but there is little overburden and no other source of backfill material or where there is little opportunity to physically replace the soils on the exposed surfaces (typically the faces and benches of hard rock quarries) (Fig. 2b); and thirdly, those also with little backfill material and where the workings are below the water-table and become water filled after mineral extraction (Fig. 2c). In the latter two cases the soils are typically mounded as surplus material or as ‘screen’ mounds that may or may not be spread later. Of course, mineral workings can have a combination of these.

Fig. 2a. Mineral workings with sufficient backfill materials for soils to be replaced

Fig. 2b. Mineral workings above the water-table without backfill and skeletal soils

Fig. 2c. Mineral workings without backfill and flooded

Recovered soils can be conserved and reused in several ways. Where the final landform and water-table rebound allows, the recovered soils can be replaced according to the pre-extraction land-use and ecosystem services and biodiversity contexts, the distribution of soil types and their profile characteristics (horizon thickness, porosity etc) having been specified at the planning and design stages and catalogued in the soil resource assessment. Here, the aim would be to replicate previous soil conditions and land uses.

However, the conservation and reuse of the soils can be an opportunity to create different conditions for enhanced productivity, biodiversity, or other soil-related services, without loss of the resource. For example, with the predicted increase in drought for soils in the east of England[29], soil thicknesses could be locally increased to improve soil water/water retention for agriculture or more resilience against flooding, whilst giving the opportunity, on areas where there will be less soil available, for biodiversity enhancement and local biodiversity recovery schemes. For example, this could provide for the creation of BNG very high distinctiveness and Biodiversity Action Plan priority acidic lowland grassland[18,30] or other highly valued habitats. This would need to be explored at both the planning and implementation stages.

There are instances, thankfully mainly in the past, where the recovery of the soils is deliberately not undertaken because it is deemed as ‘being too difficult’ (for reasons wetness, thinness, stoniness, consistency), too expensive, and even inconvenient and an expendable obstacle! This resulted in the soils being treated as overburden. Such has been the often unwarranted and short-sighted disposal of woodland and peaty soils with the loss of their ecosystem services and inherent biodiversity (fig. 3). In these cases, the quarried surfaces or overburden/waste materials have had to be relied upon as a substitute without amelioration, putting back the maturity of habitats for decades.

Fig. 3. The reuse of deep peat in peatland habitat creation

More recently, there has been a growing number of instances where overburden and quarry materials are used in preference to the natural in-situ soils, particularly those in agricultural use, for the benefit of biodiversity. This can be misguided as the in-situ soils may be better suited for biodiversity enhancement and needed to create some habitats. As a first principle, the in-situ soils should always be the preferred option unless the scheme is bespoke and there is a clear basis and an understanding of the consequences. It certainly should not be used as a calculated means of avoiding or reducing soil recovery and reuse.

For those mineral workings where there is a change in landform, such as a quarried profile, or where the water-table rebound has resulted in flooding as a water body, the pre-working soils may either have been recovered and stored or disposed of within the excavation. In the case of the former, some reuse of the soil to establish habitats may be possible by over-tipping[21], otherwise reliance would be on the longer-term natural soil-forming processes to develop ranker/rendzina soils[24,25] and the associated pioneer habitats[31], some of which are recognized to be of biodiversity value in the BNG assessment process. In the case of soils being disposed of within a flooded excavation where they make no contribution to the development of a wetland ecosystem, this should be avoided and the soils utilized in some other beneficial manner.

Time and intervention dependency

The third matter is the lack of appreciation that the persistence of some land use and habitats on the replaced soils and soil substitutes may be time limited and dependent on intervention. Without natural disturbance, such as wind, fire, flooding, or management intervention, such as grazing and cutting, the natural and universal tendency in the UK is for the vegetation to ultimately to become woodland[32]. This is largely irrespective of soil type.

The classic case is the creation of ‘flowery’ ruderal like assemblages of oxeye daisy–bird’s-foot trefoil-rich vegetation, often associated with self-establishing pioneer specialists such as the bee orchid, on overburden, quarry waste, and surfaces. In the short term these visually satisfying stands provide valuable nectar and food sources for invertebrates and birds, but without intervention, by cutting or disturbance, they are inevitably replaced by scrub and later woodland (fig. 4). Not only do these not provide a livestock grazing land use, but they may not provide for other soil-related services such as water storage and runoff attenuation. Besides, they are not recognized as being of high value for the purpose of the BNG assessment. Here, the use of recovered soils may be more appropriate and sustainable and can be managed by cutting/grazing to enable the creation of recognizable, high-value BNG habitats required to satisfy the new mandatory BNG assessment[33].

Fig. 4. Flowery vegetation established on overburden and the inevitable replacement by scrub and woodland

Hence, in practice, the land use, ecosystem services, and biodiversity potential of any natural soil or soil substitute needs to be carefully considered within the context of the medium-to-longer term, and their management dependency and its commitment. This needs to be set out clearly at the assessment stage with the honest outcomes clearly specified.

Final word

Given the changes in planning policy and environmental expectations, and the new mandatory provisions of the BNG legislation, there is a case for an urgent widening of the scope of the soil assessments and guidance for mineral workings.

Meanwhile, the industry should continue to champion soil conservation and its reuse, as promoted by the Institute of Quarrying’s issuing of guidance on soil handling[34]. For the past 50 years, the minerals industry has at every opportunity been the leader in addressing environmental matters such as biodiversity recovery[35].

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