The molecular weight of CaCO3 is 100. Thus if you put 100 mg CaCO3 in 1 liter of water and bubble carbon dioxide through the water until the CaCO3 is all dissolved you will have dissolved 1 millimole of the stuff. There will be 1 mmol of calcium and 2 mmol of bicarbonate (one from the CaCO3 and one from the CO2 that dissolved it). Because calcium is doubly charged, we say there are 2 milliequivalents of that. Because bicarbonate is singly charged there are also 2 milliequivalents of that. So there are a couple of ways we could describe this solution. We could say is 0.001 M (1 millimole) or 0.002 N (2 milliequivalents). But instead water chemists have for years preferred to refer to the hardness and bicarbonate content in terms of the amount of calcium carbonate, dissolved in natures way, that led to the observed hardness and bicarbonate (alkalinity) and so say the calcium hardness is 100 parts per million as calcium carbonate and the alkalinity (bicarbonate) contents is 100 parts per million as calcium carbonate. Thus 1 ppm as CaCO3 is 1/50 milliequivalent per liter.
The equivalent weight of bicarbonate (the same as its molecular weight because of the single charge) is 61 mg/mEq. Thus if we have alkalinity of 50 ppm as CaCO3 we have 50/50 = 1 mEq/L and 1*61 mg/L as bicarbonate. Or, put more succinctly ppm_bicarbonate = 61*alkalinity_ppm_as_CaCO3/50.
Don't know why the water industry persists in using ppm as CaCO3. It would be much better IMHO to just use mEq. And I don't know why the spreadsheets insist in asking for mg/L bicarbonate as the vast majority of water reports list alkalinity. To calculate bicarbonate precisely (the 61/50 thing is approximate) you must know pH. The alkalinity number is valid whatever the pH.
I'll get back to you on the Hegel.
[Edit] Forgot the second question: Yes, it is common practice for municipalities to publish average numbers though some also publish fairly extensive monthly data. They all have it - it's a question as to whether they want to publish it or not. Where the water is highly variable as it is in California, this can be a disaster for brewers. They must measure their water parameters every day (and Chico Brewing does this) or wind up making beers which are grossly inconsistent. This is impractical for most home brewers but it can be done. The workaround is to swamp the variations by heavy dilution with low mineral water. See http://www.homebrewtalk.com/f128/my-...15/index2.html