Every golf course superintendent knows what is in the dirt under that green. Not a suspicion, not a theory. If the green was built before 1995, the answer is a documented fact measured in milligrams per kilogram of contaminated soil that doesn’t dissolve, doesn’t degrade, and isn’t going anywhere for approximately 100 years.
At Hillcrest Golf Course in St. Paul, Minnesota, the mercury concentration in the upper two and a half feet of greenside soil reached 144 mg per kilogram. The residential cleanup standard for soil in which a child is allowed to play is 0.5 mg per kilogram. The Hillcrest greens were 288 times over that number. The fungicide that put it there was called Calo-clor. The Environmental Protection Agency canceled its registration in 1994. The mercury has been in the ground ever since.
The superintendent didn’t do anything wrong. Neither did the one before him, or the one before that. Every superintendent who applied Calo-clor to those greens in the 1950s, 1960s, 1970s, and 1980s was following industry-standard practice under a registered federal pesticide. The Golf Course Superintendents Association of America circulated it through trade publications as the go-to product for snow mold prevention at every northern-climate course in the country. The contamination is legal, it is permanent, and it is sitting under every pre-1995 course in the United States and Canada right now.
Why Mercury Became the Industry Standard
Snow mold is what forced the choice. Microdochium nivale, the primary snow mold pathogen on bentgrass, can colonize a bentgrass green in a single winter cycle when wet snow sits on turf that hasn’t fully hardened. You don’t see it under the snow. You see it in April when you pull the covers and find circular patches of collapsed, matted grass the color of straw. A superintendent who loses three greens to snow mold in a single winter doesn’t have a golf course. They have a construction project and a membership that wants answers. The pressure to prevent that outcome — season after season, course after course, from 1950 onward — is what drove mercury adoption.
Calo-clor was a combination of mercurous chloride and mercuric chloride. Mercurous chloride is the lighter compound, historically known as calomel, used in medicine through the early 20th century before scientific understanding of mercury toxicity caught up to clinical evidence. Mercuric chloride is heavier, more soluble in water, and acutely toxic at small doses. Combined at equal concentrations, the two mercury salts interacted with soil-borne pathogens in a way that researchers first documented rigorously in the early 1970s. The mechanism isn’t direct kill. Mercury retards the pathogen’s growth rate relative to the host grass, disrupting the conditions the fungus needs to establish infection. The host outpaces the colonizer. The green survives the winter.
What made Calo-clor effectively irreplaceable through most of the postwar era was the carryover effect. A fall application stayed chemically active in the root zone through the full winter. Superintendents who used it weren’t just protecting the current season. They were banking residual efficacy into the next. Peer-reviewed work published in the Canadian Journal of Soil Science in 1973 and again in 1981 documented mercury residue accumulation on treated golf course greens. Both studies found mercury concentrating in the upper 12 inches of soil. Neither study recommended against use.
That documented accumulation pattern repeats on every pre-1995 course. The carryover that superintendents valued agronomically was also carryover in the soil record. Every application added to the baseline. Every season the numbers climbed.
The 1994 Cancellation and What It Left Behind
The Federal Insecticide, Fungicide, and Rodenticide Act gave the Environmental Protection Agency authority over pesticide registration. The EPA exercised that authority on Calo-clor in 1994. The product left the market. The residue did not.
The alternatives that replaced Calo-clor after 1994 were effective. Chlorothalonil, iprodione, and propiconazole-based programs gave superintendents viable snow mold control without mercury. The chemistry was different, the residue behavior was different, but the transition required relearning application timing, relearning rates, and managing a period of uncertainty about whether the replacements would hold in heavy snow mold pressure zones. The northern tier — from Minnesota through New England and across the border into Canada — didn’t get a grace period. The cancellation took effect, the products left the supply chain, and superintendents adapted. The mercury they had put down over the previous four decades stayed exactly where it was.
The 1995 paper in the Bulletin of Environmental Contamination and Toxicology, authored by researchers from the Environmental Protection Branch at Environment Canada, is the document that closed the argument about what mercury does in golf course soil long-term. The finding was specific: mercury applied to a putting green surface concentrates in the upper 12 inches of soil, does not migrate deep, does not disperse laterally at meaningful rates, and accumulates at every treated site at concentrations that don’t naturally decline on any timeline relevant to human land-use decisions.
The Distribution Pathway Nobody Talks About
In 2001, researchers publishing in Compost Science and Utilization measured the mercury cycle at a single golf course in Manitoba, Canada, and produced the most operationally significant finding in the entire research record. At Clear Lake Golf Course, the putting green soil contained between 60.7 and 90.8 mg per kilogram of mercury — running between 121 and 180 times over the Canadian residential guideline of 0.5 mg per kilogram.
What the study added was a distribution pathway. Grass clippings from the treated greens contained between 8.53 and 20.2 mg per kilogram of mercury. The clippings absorb it from the root zone and carry it upward. The compost generated from those clippings came in at 3.16 to 6.1 mg per kilogram — four to eight times the Canadian compost quality guideline of 0.8 mg per kilogram. The compost was itself a contaminated material.
The study’s recommendation was to remove aeration cores from the site entirely rather than running them through the normal composting cycle. Aeration cores carry mercury out of the green with them. Clippings carry it out with every mow. The mercury the superintendent thought was contained in the root zone is redistributed every time someone runs a mower or a hollow-tine aerator across those greens.
The study concluded that thousands of golf courses across North America may have been treated with mercurial fungicide. It recommended a health risk assessment for the exposure levels that superintendents and grounds crews face during normal aeration and grass clipping handling. That assessment was never conducted industry-wide. Nobody told the crews running aerators over those greens what the cores they were pulling contained. The Federal Insecticide, Fungicide, and Rodenticide Act did not require remediation of residue from a legally registered pesticide used as directed. The EPA’s 1994 cancellation stopped future applications. It did not create an obligation to address what was already there.
What Happens When a Course Tries to Redevelop
The contamination stays invisible until someone tries to sell or redevelop the property. That is when Phase 2 environmental site assessment soil sampling starts. That is when the numbers appear in the public record.
At Pine Meadows Golf Course in Baxter, Minnesota, the mercury wasn’t discovered during operations. It was discovered in 2020 when the landowner — the Evangelical Lutheran Good Samaritan Society — was in the process of selling 13 acres to the city of Baxter for a stormwater wetland project. The Phase 2 assessment during that transaction turned up the contamination. Good Samaritan had purchased the property previously without finding it in a Phase 1 assessment. When the cleanup cost figures came back, they exceeded what the land was worth on the open market. The organization chose to remediate anyway. The cleanup removed 3,000 cubic yards of contaminated soil — 300 dump-truck loads — and transported it to a permitted waste facility in Becker, Minnesota. The cleanup for those 13 acres cost $300,000. Good Samaritan still owns additional contaminated acreage at the same site. Full remediation, according to the organization’s CFO, would cost millions of dollars depending on what the remaining land is used for.
The site is not eligible for Minnesota Pollution Control Agency brownfield program assistance because the contamination came from a registered agricultural product applied as directed. It isn’t legally a release, and there is no responsible party for cleanup funding purposes.
At the Hillcrest Golf Course redevelopment site in St. Paul, the St. Paul Port Authority enrolled the property voluntarily in investigation and cleanup programs through both the Minnesota Department of Agriculture and the Minnesota Pollution Control Agency. Mercury was detected in soil samples collected from former tee boxes, fairways, and greens across much of the site. The concentrations in greenside and fringe areas reached 144 mg per kilogram. Roughly half the former golf course area — not half the greens, half the entire site — requires excavation for mercury cleanup before the property can be redeveloped. The highest-concentration soils from the former greens and fringe areas require offsite disposal at a permitted landfill. Lower-concentration material can be consolidated and capped on site under a four-foot clean fill layer. That placement gets documented in an environmental covenant attached to the deed in perpetuity. Every future owner of the Hillcrest site will receive title with that covenant embedded in it.
In Pierce County, Washington, the Sundance Golf Course closed in October 2018 and entered the Washington State Department of Ecology’s voluntary cleanup program. Soil sampling identified mercury in the putting greens at concentrations up to 21 mg per kilogram. The state standard is 2 mg per kilogram, making the greens more than ten times over. Between April and August 2021, remediation crews removed 327.3 tons of soil from the 10 putting greens alone and disposed of it at Graham Road Landfill. Tee boxes, approaches, and areas near the chemical storage building were separate line items.
The Legal Framework That Keeps It Hidden
The legal mechanism the Minnesota Department of Agriculture articulated for Hillcrest applies in similar form across most state regulatory frameworks in the country. Mercury applied under a registered federal pesticide label, used as directed at agronomically appropriate rates, is not a release of a hazardous substance. It is agricultural residue. There is no enforcement action. There is no regulatory obligation for a golf course currently in operation to test, disclose, or remediate mercury in its greens.
That changes the moment the property changes use. At that point, the cleanup obligation falls on whoever owns the land when the transaction triggers a Phase 2 assessment. A superintendent who works a pre-1995 course manages no personal liability under that framework. A club or owner who tries to sell one does.
Pine Valley Golf Club opened in 1918. Oakmont Country Club in 1903. Pinehurst No. 2 in 1907. Winged Foot Golf Club in 1923. Medinah Country Club dates to 1896. The East Course — the one that has hosted more USGA championships than any other course in the country — opened in 1912. Every one of those clubs operated through the entire Calo-clor era, using the same industry-standard products on the same types of bentgrass greens for the same reasons: snow mold prevention, consistent surface quality, season-to-season reliability. The peak application period ran from the 1950s through the early 1990s. None of those clubs has been publicly tested for mercury in their green soil. None of them has redeveloped. None of them has triggered the Phase 2 process that would put soil sample data into a public record.
The 100-Year Clock Running Under Every Course
The Canadian research from 2001 estimated 100 years of passive remediation time to bring contaminated green soil down to guideline levels. The oldest surviving greens from the Calo-clor era haven’t had 100 years to remediate. They are 30 years out at most, assuming a superintendent stopped using mercury products the moment the registration was cancelled in 1994. The mercury in the soil under those greens has nowhere to go and nothing to pull it out except the aeration core process — which redistributes it rather than removes it.
Every golf course superintendent working a pre-1995 course understands this at some level. The agronomic record is available. The regulatory framework is specific about when it matters and when it doesn’t. The job is to manage the surface the membership walks on and make it as close to perfect as the budget allows. The chemistry under that surface doesn’t require action unless the property changes use.
The superintendent knows the greens run fast. The mercury doesn’t move. The course you are playing, if it opened before 1995, has the same chemistry in the root zone. The superintendent managing it today inherited the decisions made by the superintendent 40 years ago. The product was legal. The application was correct. The result is a permanent legacy in the soil that the modern superintendent cannot remediate, does not have to disclose, and manages around every single day — in the aeration schedule, in the composting decisions, in the way the cores get handled. All of it now informed by a chemical history that the club itself may not know in detail, because no one has looked.