I've seen an oily sheen on water from what I thought was decaying material that washed in to the water too.

Personally? I wouldn't lose any sleep over it. The % of creosote probably isn't high enough to worry about. But, if getting the water tested will let you sleep easier, get it done!!

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3/4 to 1 1/4 ac pond LMB, SMB, PS, BG, RES, CC, YP, Bardello BG, (RBT & Blue Tilapia - seasonal).