PM me with an email addy and I can send you monthly .xls chemical analysis files from the OKC Water Lab all the way back to, IIRC, Jan 2007.
You just gotta know who to call.
Trouble is, due to last years budget cuts, they've also trimmed down the test results to a small handful of minerals/ions. Once you've seen the historical data you will be able to figure a reasonable average for the month (useful because our lakes turn seasonally and also useful for the wetter months, when those do come).
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