Contaminants in saprolite: an overlooked hotspot of environmental concern PhD

Increasing attention is being placed on the transfer and storage of contaminants in soils, including microplastics, pharmaceuticals (PPCPs), bacteria, and hydrocarbons (PAHs). Recent research has developed methods to detect the presence of these contaminants in, and track their movement through, the soil profile. However, much of this work has ignored the underlying parent materials (saprolite) from which soils are formed. This fully-funded NERC CENTA PhD Studentship (3.5 years) represents a ground-breaking project using a combination of digital mapping, laboratory analyses, and field-based experiments to assess the extent to which contaminants are accumulating in saprolite. It will be an exciting opportunity to gain valuable experience in the waste management sector. CENTA is a consortium of Universities and research institutes that are working together to provide excellence in doctoral research training within the remit of the Natural Environment Research Council (NERC). Successful home-fees-eligible candidates will receive an annual stipend, set at £15,609 for 2021/22, paid directly to the student in monthly increments, full university fees, and a research training support grant (RTSG) of £8,000.

Project Highlights:

•A ground-breaking project to assess the extent to which saprolite is a reservoir of contaminants, and a potential hotspot for environmental concern. 

•A combination of digital mapping, controlled laboratory analyses, and field-based experiments.  

•An exciting opportunity to network with industry, gain valuable experience in the sector, and enhance future career prospects.

Overview

With millions of tonnes of sewage sludge being spread on farmland every year, increasing attention is being placed on the transfer and storage of contaminants in soils, including microplastics, pharmaceuticals (PPCPs), bacteria, and hydrocarbons (PAHs). Recent research has developed methods to detect the presence of these contaminants in, and track their movement through, the soil profile. However, much of this work has ignored the underlying parent materials, from which soils are formed.

Typically, soils overlie bedrock, of which the uppermost zone comprises ‘saprolite’. Saprolite is categorised as physically intact but chemically weathered bedrock, meaning that it is undergoing the process of losing its cementing agents, but retains the volume and fabric of the parent rock. As such, it sits as a boundary layer between the soil profile and unweathered bedrock. The presence, depth, thickness, and properties of the saprolite horizon are variable, and are dependent on both the bedrock lithology and soil management practices. 

We currently have little or no knowledge about the extent to which the saprolite acts as a reservoir for contaminants, the types of contaminants that may pass through to this zone, the rates at which they accumulate in saprolite, the mechanisms that may accelerate their accumulation, and their short-to-long-term fate. This constitutes a major gap in our knowledge and understanding of the transport of mobile contaminants in soil systems. Moreover, as saprolite weathers to form soil, contaminants accumulating within saprolite could be remobilised back into the soil, representing a ticking time-bomb for ecosystems and the wider environment.

This project will be one of the first to assess the saprolite as a potential hotspot for environmental concern. It will systematically review the state of our knowledge around the interactions between soils and saprolite, and use digital mapping techniques to map the extent and thickness of saprolite across the UK. In a series of laboratory-controlled experiments, the transfer of different contaminants from soils to saprolite will be investigated, and new field-based techniques will be developed and deployed to detect and measure contaminants in saprolite.

Methodology

Phase 1: State of the art: Existing knowledge and gaps about the interactions between soils and saprolite will be reviewed, including weathering rates, solute leaching, chemical changes, and bioturbation. This phase will help scope out laboratory testing. 

Phase 2: Digital mapping: The student will use digital mapping to map the extent and thickness of saprolite for contrasting UK lithologies. This phase will help identify the dominant types of saprolite and inform subsequent laboratory analyses. 

Phase 3: Soil mesocosm: A series of laboratory-controlled mesocosm experiments will be carried out to characterise the physicochemical properties, geochemistry, and mineralogy composition of contrasting saprolites being exposed to chemical contaminants. Advanced screening analytical techniques (e.g., GCMS, LCMS, XRF, FTIR) will determine the chemistry present, and the fate of these chemicals across the soil-saprolite boundary.

Phase 4: Field validation: The student will collect cores to measure the presence and concentration of different contaminants down these profiles.

Partners and collaboration

British Geological Survey will be co-supervisors in this Collaborative Studentship. As part of a six-month placement, near the start of the PhD research, the BGS will provide the student with essential access to the National Geoscience Data Centre records on boreholes, geotechnical properties datasets, BGS Geology mapping and expertise on digital geoscience and geology.  These components will be used to create a workflow for digital mapping of the expected extent of saprolite in the UK. 

Possible timeline:

Year 1: (M1-3) Aims, objectives, and research questions will be finalized; (M4-9) systematic literature review will be started (i.e., search string confirmed, abstracts screened, data extraction); (M10-12) systematic review completion (i.e., analysis written). 

Year 2: (M13-18) Six-month digital saprolite mapping at BGS; (M19-24) Controlled laboratory experiments at Cranfield.

Year 3: (M25-M29) Field-based experiment; (M30-M36) Writing up and compiling thesis.

Sponsored by NERC through CENTA DTP, Cranfield University

The project is open to all applicants who meet the academic requirements (at least a 2:1 at UK BSc level or at least a pass at UK MSc level or equivalent). Please note the grant covers fee costs for a Home award. Unless you are eligible for such a Home award, you will need to consider how you will be able to meet any shortfall in funding for tuition fees, e.g. self-funded. Please contact the supervisors listed on the project for more information.

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