From the EPA.gov website:

Spent Foundry Sand

Basic Information
Spent foundry sands are generated by the metal casting industry. Foundries purchase new, virgin sand to make casting molds, and the sand is reused numerous times within the foundry. However, heat and mechanical abrasion eventually render the sand unsuitable for use in casting molds, and a portion of the sand is continuously removed and replaced with virgin sand. The spent foundry sand is either recycled in a non-foundry application or landfilled. It is estimated that less than 30 percent of the 10 million tons of spent foundry sands generated annually are recycled. The Agency believes a greater percentage of spent foundry sand can be safely and economically recycled.

Beneficial Use
The EPA has found that silica-based spent foundry sands produced by iron, steel, and aluminum foundries can be safely reused to save energy, reduce the need to mine virgin materials, and reduce costs for both producers and end users. The EPA supports the use of silica-based spent foundry sands from these foundry types in the following applications:

As an ingredient in manufactured soil;
As an ingredient in soil-less media (potting soil);
As a foundation layer of roads (subbase).

Foundries and foundry sand recyclers should consult state regulators to ensure that planned uses are consistent with state beneficial use and waste management programs and that the chemical and physical properties of the sand meet applicable state environmental limits, engineering performance criteria, and other state requirements.

Risk Assessment
The EPA, in conjunction with the U.S. Department of Agriculture (USDA) and the Ohio State University (OSU), launched a collaborative effort to evaluate the potential risks of using silica-based spent foundry sands produced by iron, steel and aluminum foundries, and to encourage beneficial use. The risk assessment focused on soil-related applications; specifically manufactured soil, soil-less potting media, and as subbase, which is a foundation layer of roads. The overall goals for the risk assessment were to:

Review the available information on spent foundry sand in soil-related applications

Identify likely exposure pathways and receptors associated with various uses

Use a combination of screening and modeling methods to determine whether the proposed uses of spent foundry sand are protective of human health and the environment and

Discuss the findings within the context of certain overarching concepts (e.g., the complexities of soil chemistry) and provide conclusions.

Based on the results of the risk assessment, the EPA and the USDA support the beneficial use of these materials that would otherwise go to waste, because the constituent concentrations found in silica-based spent foundry sands from iron steel and aluminum foundries are below the agency’s health and environmental benchmarks. Any conclusions drawn by the risk assessment should be understood within the limitations and scope of the evaluation, including the following:

Only silica-based spent foundry sands (SFS) from iron, steel and aluminum foundries are evaluated in the risk assessment. In contrast, spent foundry sands from leaded brass and bronze foundries are often regulated as RCRA hazardous waste. SFS from non-leaded brass foundries and SFS containing olivine sand also are not evaluated in the risk assessment.

In addition to SFS, foundries can generate numerous other wastes (e.g., unused and broken cores, core room sweepings, cupola slag, scrubber sludge, baghouse dust, shotblast fines). The assessment, however, applies only to SFS as defined in the assessment: molding and core sands that have been subjected to the metalcasting process to such an extent that they can no longer be used to manufacture molds and cores. To the extent that other foundry wastes are mixed with SFS, the conclusions drawn by the assessment may not be applicable.

Environmental Benefits
Approximately 2.6 million tons of spent foundry sand is beneficially used outside of foundries annually, with iron, steel and aluminum sands representing 96 percent of the foundry sands that are beneficially used. Currently, only about 14 percent of those sands are beneficially used in soil-related applications. The EPA believes that there is potential for substantial growth in the beneficial use market for the applications studied in the risk assessment, resulting in increased environmental benefits. The EPA estimates the environmental benefits from using silica-based spent foundry sand in the specific applications studied in the risk assessment, at the current use rate, results in the following savings in one year:

The energy savings equivalent to the annual electricity consumption of 800 homes;

CO2 emissions reductions equivalent to removing 840 cars from the road for one year; and,

Water savings of 7.8 million gallons.

The risk assessment supports the EPA's ongoing efforts to advance Sustainable Materials Management by demonstrating that the beneficial use of spent foundry sand in the soil-related applications evaluated are protective of human health and the environment and yield environmental benefits.

Risk Assessment Documents
Please go to the following EPA.GOV page for the supporting documentation:
https://www.epa.gov/smm/frequent-questions-about-epas-risk-assessment-spent-foundry-sands-soil-related-applications