diff --git a/R/import_functions.R b/R/import_functions.R
index e44a5a6..3690fe6 100644
--- a/R/import_functions.R
+++ b/R/import_functions.R
@@ -894,6 +894,18 @@ read_contaminants <- function(file, data_dir = ".", info) {
       colClasses = var_id[ok]
     )
     
+    # additional validations
+
+    # every valid `uncertainty` must have a valid `unit_uncertainty`
+    uncertainty_present <- which(complete.cases(data$uncertainty))
+    uncertainty_present_valid_units <- 
+      data$unit_uncertainty[uncertainty_present] %in% c("%", "U2", "SD")
+    if (! all(uncertainty_present_valid_units)) {
+      stop(
+        "Missing or invalid uncertainty units for specified uncertainty values. ",
+        "Please check that all uncertainty values have a valid unit: %, U2, or SD"
+      )
+    }
   }