I believe this is "The Case of the Gring's Mill Goblin" by Thomas R. Dulski. It was apparently only published in Analog, December 1985. Some of the points are not quite right, but are very close and taken as a whole suggest this is the story you're looking for.
The detective's office was in King of Prussia, PA, and he said that Lititz in Lancaster county was 60 miles east of King of Prussia.
Dexter Woodside works as an industrial researcher for Monash Chemical in King-of-Prussia, where he lives in a split-level with his recent wife. He is braced on his way home one day by Dr. Oliver Baker, a former employer, to join him on a case that is taking him to Lancaster, not Lititz:
Lancaster is about 70 miles east of King-of-Prussia—hardly in my own backyard—but such distinctions could hardly be expected to carry any weight with Baker. The point was that I mentally resolved not to enmesh myself with one of Baker's cases again, no matter what.
There was also an assay office in Lititz.
It wasn't a formal assay office, but a local testing company run out of a converted house in Lititz.
I started to speak, but Baker raised his hand. "What was the name of the laboratory that performed the analysis?"
Myron scratched his beard. "Lititz Testing, I guess it was. That's only a little ways from here. They're small. They mostly specialize in soil and water analysis. But they've got some pretty modern equipment so I don't doubt that they did a good job."
