A recent SCS Engineers project demonstrates what can be achieved when cleanup decisions reflect actual site conditions rather than default assumptions.
At a 66-acre residential redevelopment site, arsenic concentrations exceeded the default residential risk-based screening level. Using that level as the cleanup endpoint would have required widespread soil removal and importing clean fill, potentially making the project economically unworkable.
Instead, SCS Engineers developed site-specific cleanup target levels based on the property’s planned and enforceable future-use conditions. The resulting strategy maintained all human health protections, achieved regulatory closure, and saved the developer more than $5 million in remediation costs.
The project illustrates an important distinction: screening levels are valuable starting points, but they are not always appropriate cleanup endpoints.
How RBSLs Become Remediation Mandates
Risk-based screening levels, or RBSLs, help practitioners evaluate potential human health risks at contaminated sites and determine whether further investigation is warranted. Regulatory agencies publish these values in lookup tables, such as the U.S. Environmental Protection Agency’s Regional Screening Levels.
These tables provide a fast, consistent way to screen detected chemical concentrations. The problem arises when screening levels are applied automatically as remediation requirements or cleanup target levels, known as CTLs.
RBSLs are calculated using standardized equations and conservative assumptions intended to cover broad populations and land-use categories. Those assumptions work for initial screening, but they may not reflect how people will actually use a particular property.
When that distinction is overlooked, conservative screening assumptions can lead to unnecessarily extensive remedies, higher costs, project delays, and lost redevelopment opportunities—without necessarily providing additional protection.
A site-specific evaluation can indicate whether the default assumptions fit the property. When they do not, practitioners should be prepared to propose and defend more appropriate values using sound science, transparent analysis, regulatory coordination, and enforceable controls.
When to Consider Site-Specific CTLs
Site-specific CTLs are not appropriate for every project. They require sufficient time, technical support, regulatory authority, and reliable information about future site use. Before pursuing a site-specific approach, development teams should evaluate four practical considerations with the expertise of their environmental engineer:
Project timeline: Developing site-specific CTLs and obtaining regulatory approval can take months. Projects with aggressive schedules may not have enough time for the necessary analysis, documentation, and agency coordination.
Project budget: Risk assessment, data analysis, and regulatory coordination require an upfront investment. Weigh those costs against the remediation expenses a site-specific approach could avoid.
Regulatory authority: The overseeing agency must have legal authority and an established mechanism for approving site-specific CTLs. Early communication with regulators is essential to confirm whether the approach is viable.
Site complexity: Properties affected by numerous chemicals, multiple environmental media, uncertain future uses, or poorly understood exposure conditions may require a comprehensive site-specific risk assessment. They may not be suitable for streamlining adjustments to individual exposure assumptions.
The Technical Basis for Site-Specific CTLs
RBSLs are derived using default input parameters that fall into three categories: exposure parameters, toxicity values, and target risk or hazard levels. Site-specific CTLs are developed by modifying the input parameters in one or more of these categories, as described below.
Exposure Parameters
Exposure parameters describe how and to what extent people may encounter chemicals at a property. They include factors such as:
Default screening levels generally rely on standardized residential, commercial/industrial scenarios. Many properties, however, do not fit neatly into those categories.
Public parks, schools, residential developments with enforceable occupancy or activity restrictions, and automated warehouse or distribution centers may have exposure patterns that differ from the default scenarios. Where those differences are well documented and expected to continue, site-specific parameters may provide a more accurate basis for establishing cleanup levels.
Any revised assumptions must be supported by reliable evidence and, where necessary, enforceable institutional or engineering controls.
Toxicity Values
Toxicity values describe a chemical’s potential to cause adverse health effects. These values generally come from EPA-endorsed databases and may change as new toxicological information becomes available.
Modifying an EPA-established toxicity value is typically difficult to justify. Project teams should nevertheless confirm that the risk evaluation uses the most current agency-approved values.
Target Risk and Hazard Levels
The target risk or hazard level is the maximum acceptable benchmark used to evaluate potential human health effects.
For noncarcinogenic chemicals, the benchmark is intended to keep exposure below a threshold associated with adverse effects. For carcinogens, it is expressed as an acceptable increased probability of developing cancer.
Some regulatory programs allow site-specific evaluations to use an acceptable risk range rather than a single conservative target. Where regulations permit this flexibility, an alternative cleanup level may remain within established human health standards.
Case Study: A 66-Acre Residential Redevelopment
SCS Engineers was retained to assess and remediate approximately 66 acres planned for residential redevelopment. The property had historically been used for agriculture and later operated as a golf course.
Arsenic associated with the historical, legally permitted application of agrochemicals was detected in soil at concentrations above the default residential RBSL. Remediating the entire property to that level would have been economically unrealistic and could have prevented the redevelopment from proceeding.
SCS evaluated whether the default exposure assumptions accurately reflected the property’s planned and enforceable future-use conditions. The team then developed site-specific CTLs by modifying two components of the exposure analysis.
Accounting for the Applicable Resident Population
The default residential arsenic RBSL incorporates exposure assumptions for all demographics, weighted across ages from birth through adulthood.
Estimated childhood exposure is higher than adult exposure because children generally have higher soil-ingestion rates relative to their body weight and different behavioral patterns.
For this project, enforceable occupancy conditions could safely eliminate the child exposure pathway. SCS therefore removed the childhood component from the applicable exposure calculation. This reduced the estimated exposure dose and supported a higher CTL while maintaining the mandated level of human health protection.
Because this adjustment depended on future occupancy conditions, those conditions had to be clearly defined, legally enforceable, and maintained over time.
Establishing Separate Exposure Units
SCS also evaluated how activity patterns and potential soil contact would vary across the property. The planned development supported two distinct exposure units:
Separating the property into these exposure units allowed the cleanup levels to reflect anticipated activities within each area, rather than applying one assumption uniformly across all 66 acres.
The analysis supported a common-area CTL two times higher than the default residential RBSL and a residential-lot CTL four times higher than the default.
The assumptions underlying the risk assessment were formalized through Homeowners’ Association agreements and restrictive covenants. These enforceable controls allowed the property to achieve regulatory closure without the added cost and disruption of widespread soil removal and clean-fill importation.
The strategy saved more than $5 million while keeping the project compliant with applicable regulatory benchmarks for human health protection.
Recognizing the Limits of CTLs
Site-specific CTLs are not used merely because the default level is expensive or inconvenient. Credible data, realistic assumptions, regulatory approval, and controls that remain effective for as long as needed must support an alternative value.
A site-specific approach may not be appropriate when:
Acknowledging these limitations strengthens the approach’s technical and regulatory credibility. The goal is not to find the highest defensible cleanup level. It is to identify the level that accurately reflects site conditions while meeting the required standard of protection.
A More Deliberate Starting Point
Default screening levels remain essential tools for evaluating contaminated properties. They provide consistency, efficiency, and an appropriately conservative starting point.
They should not, however, become automatic cleanup endpoints without first considering whether their underlying assumptions fit the site.
Before accepting a default screening level as the remediation target, project teams should ask:
When the answers support further evaluation, a site-specific CTL can align the remedy more closely with actual risk, protect human health, and preserve viable redevelopment opportunities. Work with your environmental engineer or consultant skilled in due diligence, remediation, risk management, and federal/state/local agency rules for the best protection of the community and your development.
Default levels are a starting point—not necessarily the final answer.
Meet our Author: Lisa Smith serves her clients as a Senior Technical Advisor and the SCS National Expert in risk mitigation and risk assessment in corrective action. Listed below are resources and more articles that may be of interest.
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