I don't have experience building a probe, but i have a lot of experience using hydrolab and YSI devices in ways that are not effective. These devices cost over $1000. And so, they are cost-prohibitive for non-profits, and shape official government response monitoring practices in ways that retard effective sampling.
We have a need to monitor oxygen demand independently of the paper mills, who monitor their own waste for LA DEQ in Discharge Monitoring Reports. LA DEQ has little to no response capacity, so during these recurrent industrial catastrophes, we need to monitor the river, and be able to do it quickly.
There is also a need to monitor chronic leakage from legacy waste pits. Salt is a good indicator of these pits, which are filled with used drilling muds. Drilling muds usually contain BTEX and NORM materials with the brine.
For Oxygen, it would be appropriate to drop sensors with loggers, rather than boat around spot checking with handhelds. Spot checking is cheaper from an equipment point of view but bleeds away gas and limited staff time.
An oxygen sensor is calibrated based on temp and conductivity; andconductivity gives you one way of measuring salinity
so, commonly, handheld field probes (like YSI 85) have temp, salinity, conductivity, and oxygen. YSI 85 is a standard scientific probe for fisheries work.
These are also common pollutants of concern. ranked by priority:: nitrate / ammonia enterococcus / e coli PAHs and phenols from oil waste and paper mill effluent (also strange organic chemicals like Sylvic acid) Hg, Ar, Cu, Pb, other metals Salt ::brine from old drilling muds from re-mobilized oil waste pits
I'm glad to hear discussion of an oxygen sensor. Whether we're fighting faulty water treatment or monitoring paper mill effluent, oxygen is often something we, as watchdogs, want to measure. unfortunately it's very variable, and we often are not in the right place at the correct time.
Often we can't measure or can't make sense of spot measurements. LA DEQ can't, either. many person hours and fuel were burned in monitoring oxygen on the river in response to recent fish kill in Bogalusa, for example, because the agency uses a handheld probe and runs up and down the river in a boat taking readings. example reports (without time of day) are here.
But what we really want to get is repeated measures of early-morning DO at depth over a series of days.
Knowing the daily variation is also useful. LA DEQ does not wake up early, and neither do we, honestly. a logger would be best, a cheap logger would be necessary.
Fisheries research depends on repeated spot measurements over years to get a picture of the oxygen conditions on the river. In release situations, we do not have years to build up a profile of oxygen in the river at a point over time.
LA DEQ also does not take readings at high water, due to boating safety precautions. And yet, there are a certain types of toxins (like sylvic acid from paper mills) that are activated only during or soon after high water events that mobilize bottom sediments. If something nasty in sediments gets re-mobilized from the river bed during high water, we don't see it or any local effects.
WATER TREATMENT There are similar problems downstream of watertreatment ponds; the problems are more chronic--as the ponds age, they fill and become useless. But our sampling challenges are similar.
NON PROFIT NEEDS A non-profit that would want to collect data would need to do so with less money and staff. rather than running two boat trips on 40 river miles a day for 6 locations ("stations"), it would be cheaper to drop six probes in place for a couple of days at a time, and just data collection / battery recharge boat runs on volunteer time.
From experience with stream loggers in Georgia, I wouldn't want to leave probes out for long periods. probes are tampered with or are more likely to be displaced by multiple high water events. So the time period is similar to a fishing rig. Also, probes that sit for a while get gunked up, this is the problem with the water quality stations in city park. algae.
It would be excellent to have droppable oxygen probes, weighted to the bottom of the river, buoyed for placement in the water column, but secured to the bank by a second line. attached is a dream diagram. the bottom probe is essential. Having multiple drop stations makes the equipment cost ring up quickly, so one bottom probe in many places would be preferable to half the locations but probes at two depths.
This kind of deployment is crucial in our recurring crises, because it takes LA DEQ weeks to respond to a problem. But we could also use a similar protocol to get a weekly profile during "typical" seasons or high water periods, to establish a record of what the river should be, so we can track long term changes.
Problem: stable oxygen probes require water agitators, like the one on the hydrolab multi-probe. If the current is quick enough (1 foot per sec), it ceases to be a problem, but periods of slack water are the times when oxygen will be low, which is the event we want to capture and know the frequency of.
Another problem would be calibrating electrochemical probes, which would eat up a lot of time before deployment, compared to handhelds. still, it seems worth it, given the time saved not running up and down the river over days.
Again, This information applies to the electric/ membrane style of oxygen sensor that I am familiar with, and is relevant to common dilemmas I face as part of community-based organizations trying to learn about our rivers, and to protect and improve them.
proprietary equipment i have used (besides YSI) Hydrolab multi-probe Sondes
Scott Coastal Wetland Specialist, Gulf Restoration Network