I have gotten similar readings from my tap water before, it was many years ago. At the time I didnt know that the max allowed by the EPA in drinking water is 10 ppm. Shortly after that the water co issued a warning that infants and pregnant women shouldnt drink the water. I think they said something like the nitrate level was "approaching" safe limits. Maybe their tests are different than ours or the nitrate level is increasing in the pipes. Or maybe they were lying.
By the way, they don't put nitrate in the water, they just dont remove it. It comes from the water source. Many of our lakes, rivers and groundwater sources have high nitrate levels from time to time. Some is natural, like from rotting leaves in the fall, and some is from man made pollutants, like fertilizer runoff.
Unless you have especially sensitive fish, I wouldnt panic over 40 ppm in your tanks. Most sources consider 40 safe, and nitrate is more of a long term problem. Even very high levels (like 200+) of it doesnt kill immediately like ammonia or nitrites. It gradually stresses the fish, making them more succeptable to diseases.
I'm interested to hear what the water co tells you, if anything.