New regulatory requirements addressing water quality have significant impacts to Department of Defense (DoD) installations. Distributed measurements of environmental parameters are needed to meet Total Maximum Daily Load (TMDL) requirements and to gauge sources and dynamics of pollution in large water bodies. Although several sensors have been developed and deployed for the common environmental parameters (dissolved oxygen [DO], turbidity, and pH), the systems tend to be bulky, power-intensive, and too expensive for deployment in large numbers for distributed measurements. In addition, commercially available sensors do not perform satisfactorily beyond a couple of weeks of continuous field deployment because of excessive biofouling. Employing sensor platforms with multiple sensors in a miniaturized form capable of long-term use could relieve many of the limitations experienced with current multisensor monitoring technology.
The objective of this SERDP Exploratory Development (SEED) project was to provide proof of concept for an inexpensive, field-deployable sensor platform equipped with three recently developed miniature optical sensors for DO, turbidity, and pH.
The platform consists of three major parts—sensing foils, optoelectronics, and radio communication. The sensing foils for DO and pH are created by immobilizing certain elements (e.g., phosphorescent dye and modified fluorescent ratiometric pH indicator). Each foil has a permeable light shield and consists of an antibiofouling layer. The sensing foil readout is performed by optoelectronics consisting of light emitting diodes (LED) for DO and pH. The turbidity sensor is an optical setup fixture with a fixed-length lightpath and operates as a standard absorption photometer. The reading of this sensor measures the decrease of the light intensity upon passing through an analyzed medium, using only a detector filter to avoid saturation of the photodiode by the ambient light. The measured light intensities are converted in voltages, amplified, and synchronously detected. The resulting signal is digitized by an analog-to-digital (A/D) converter. The platform then establishes radio communication with the base station and transmits the data every 15 minutes. At the base station, the received data is uploaded into a computer for further use.
Using these reliable, inexpensive, and low-power sensors, DoD land managers will be able to assess and address TMDL sources on installations. The sensor platform is designed to communicate collected data, covering a range of 1 kilometer, to a central server through a wireless network for a period of up to 1 month. The sensors are based on a common principle (fluorescence) and share the optoelectronics for detection, ensuring the expandability of the platform toward sensing other pollutants when novel detection chemistries become available. (SEED Project Completed - 2007)