POTENTIAL IMPACT TO MOLLUSCAN COMMUNITIES OF FLORIDA’S WAKULLA SPRING DUE TO HEAVY METAL POLLUTION FROM COINS

Coin tossing has been historically documented in many bodies of water. Wakulla Spring provides base flow to the Wakulla River in northern Florida and has been used as a wishing well since the 1850s. Thus, thousands of coins lie at the bottom of the spring. These coins, mostly pennies, contain copper and zinc, which may pose contamination risks to freshwater ecosystems. Mollusks are vulnerable to heavy metal bioaccumulation, which potentially contributes to the decline in diversity observed by comparing live, dead, and fossil assemblages in the spring. This study uses a conservation paleobiological approach to examine these concerns.

To assess heavy metal toxicity, the potential amount of copper lost from pennies was tested by quantifying mass lost while underwater. Hundreds of pennies were collected as bycatch from the bottom of the headspring during sampling by SCUBA. Coins were cleaned using sonication and 473 ml of vinegar. Mint dates and weights were recorded. Initial weights, as reported by the U.S. Treasury, were compared to post-cleaning weights. Penny dates were identifiable for 617 coins ranging from the 1920s to 2000s (mode 1960s). 584 pennies were copper-based (minted prior to 1982), and 33 were zinc-based (1982 or later). Copper and zinc losses per coin varied by decade, ranging from 3-50 grams per decade. Cumulative copper loss (total pennies per year) peaked in the 1950s and 1960s. During those decades, more than 50 cumulative grams of Cu or Zn were lost, representing 0.2743 grams lost per average coin, posing a potential contamination risk.

Pennies appear to corrode more over time in the spring and are expected to corrode faster with more saltwater intrusions. Zinc-based pennies minted starting in 1982 appeared more susceptible to corrosion than older copper-based coins. A significant decline in pennies was observed starting in the 1980s that could reflect increased post-1982 corrosion, fewer pennies being tossed in, or both. If the former is true, even more zinc may have entered the system since 1982; if the second is true, less zinc is entering the system. Overall, substantial amounts of copper and zinc from pennies may be entering this aquatic ecosystem in the Wakulla River. We hypothesize that declines in mollusk communities may be due to point-source heavy metal pollution by seemingly harmless human activities.