This is the fourth monthly progress report on the TMDL project in sunny Waimanalo. The rainfall in June totaled 0.93 inches, about 2/3 of average. The rain of May 16-17 continues to be by far the biggest event since the project started.

I began analyzing the dry weather samples and the samples from the May 16-17 rain. No sooner had I begun than our analytical balance developed a problem. We had to ship it back to the mainland for repairs. Before that happened, I completed the phosphate analyses. I am enclosing those results along with the other analyses that have been completed to date.

The phosphate results make an interesting comparison with the suspended solids (SS) data. The phosphate concentrations during the storm of May 16-17 were roughly 10 times higher than the dry weather results. The only exception is station 1, where the concentrations were about equal. The SS concentrations during the storm were about 100 times higher than the dry weather SS numbers. Speaking in order-of-magnitude terms, the storm was equivalent to about 10 days of dry weather flow in terms of phosphate flux and about 100 days of dry weather flow in terms of SS flux.

I plan to analyze for silicate (Si) next. I am hoping that the silicate results may provide a crosscheck on our water flux numbers. My reasoning is that the additional runoff from the rain will contain virtually no silicate. Hence I expect to see a drop in silicate concentration corresponding to the percent contribution of rainwater to the discharge. As you are probably aware, groundwater in Hawaii is loaded with silicate. Typical concentrations are 300-500 mM. Whether the silicate concentrations are useful or not for estimating flow remains to be seen. Basically the idea would be to estimate flux during a storm from the equation

Flow during storm= dry weather flow x (dry weather Si concentration / storm Si concentration)

This equation assumes that silicate is conservative during a runoff event. I think that is probably a safe assumption.

As you may recall, we added station 23 to the experimental design just before the May 16-17 storm. Lisa Ferentinos collected a dry weather sample there about a week ago. However, by that time our balance had been sent back to the mainland, so I have no data on that sample except for turbidity, which was not surprisingly quite low, about 2 NTU. Station 24 is also in the experimental design, but so far there has been no flow there.

Lisa told me she has a picture someone took during a big rain a couple of years ago. I have not seen the picture, but it reportedly shows water overflowing the bridge at station 16. I would settle for a lot less than that right now. The next few months will probably be very dry.

[submitted to EPA, July 21, 1999]