There's a little stream on the property I've known since I was a kid. Small waterfall, prairie on both banks, water clear enough to count pebbles. Frogs everywhere the day I filmed this, and a fish holding in the pool below the falls — too big for a creek chub, so I'm betting it's a bluegill that washed down from the pond above.
About as pretty as water gets in central Iowa. It still tested 2 to 5 ppm nitrate.
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A pond is a biological sponge. Nitrate is plant food, and algae and submerged plants pull it out fast. Water sits there for weeks. Down in the sediment, denitrifying bacteria convert what's left to nitrogen gas.
A stream doesn't get that chance. Water moves through — whatever comes in goes out the other end. This one has no cattail fringe, just a narrow channel over rock. Nothing to slow it down, nothing to eat the nitrogen on the way by.
There's a field up in the watershed, and that's the obvious source. But probably not the way you'd think. The prairie buffer catches overland flow — and in Iowa, a lot of water doesn't travel overland. It travels through drainage tile four feet down and comes out a pipe. Tile flow skips every buffer you plant.
I'd been saying the pond reads zero. Then I pulled up the actual record.
May 23: right at 10 ppm. June 19: above it. September 9: 0.1 ppm.
So it isn't a pond that stays clean. It's a pond that was at the drinking water standard in early summer and is nearly empty of nitrate now. And I can think of two explanations that fit that curve equally well.
One: the pond is doing the work. Warm water, an algae bloom, nitrogen getting consumed. The September test came back cloudy, which is what that looks like.
Two: the watershed just stopped delivering. May and June are the peak of Iowa's nitrate curve — spring application plus heavy tile flow. By September the tile has gone quiet and the crop has taken up what it's going to take up. Nothing is arriving.
Same chart. Completely different story. If it's the first, the pond is an asset that's removing nitrogen. If it's the second, the pond is just empty because the tap turned off, and it'll load right back up next May.
The way to tell them apart is to test the inflow on the same day as the pond. If the inflow is also near zero, the pond isn't removing anything — it's just not receiving.
One reading tells you what one spot looked like on one afternoon. Nitrate spikes after rain, drops in dry stretches, and runs completely different in June than in October. I have 82 days between those last two pond tests, and the line between them is drawn, not measured. Next spring I'm testing every two weeks through May and June, when the pulse actually happens.
One note on strips: check whether yours report nitrate as nitrogen (NO₃-N) or as the nitrate ion. The same water reads about 4.4x higher in ion units, which matters a lot if you're comparing against that 10 ppm standard.
Everything goes into Landwright with a date and a location. I'll be back at this creek in November if there's still water in it — probably with a kid and a fishing pole, to go see about that bluegill.