## 1 Introduction  
In terms of natural resource preservation, a real-time and constant environmental monitoring system is an important research and development topic [1; 2; 3; 4; 5; 6]. The targets of the environmental monitoring systems are very far-reaching and can include air, water, and soil. In this paper, we specifically focus on water quality monitoring, which is one of the most important issues worldwide. Water quality is closely related to the quality of drinking water and water for fishing and agriculture. In typical water quality monitoring systems, physical and chemical quantities such as pH, turbidity, dissolved oxygen concentration, chlorophyll \(a\) concentration, temperature, salinity, and pressure are monitored. However, a commercially available conventional water quality monitoring system is generally large and expensive. The high cost of monitoring systems prevents the real-time and continuous monitoring of a large area. In other words, sufficient numbers of water quality monitoring systems cannot be established in large natural resources such as lakes or rivers due to their high cost. It also causes low accuracy of water quality monitoring.  
In order to obtain a low cost and high accuracy for lake or river water quality monitoring systems, we have been developing a wireless sensor network system. Our wireless sensor network system consists of a sensor module and a system module, which includes a wireless transceiver module,control ICs for sensors, and a power module. Previous partial development results on the nodes of wireless sensor network systems have been reported in [7]. On the other hand, in the case of the sensor module, various sensors to monitor water quality, such as pH sensors, turbidity sensors, chlorophyll \(a\) concentration sensors, and temperature sensors, will be integrated into a microfluidic device. Among those sensors, we have developed a pH sensor. Our microfluidic device-based pH sensor measures pH with three electrodes: working, counter, and reference electrodes. We integrated three electrodes into the microfluidic channel, including a reference electrode. This can be applicable to not only water quality monitoring but also other fields such as nanoparticle synthesis monitoring [8]. Before integrating the turbidity and chlorophyll \(a\) concentration sensors into the microfluidic sensor module with a pH sensor, we studied the feasibility of simultaneous sensing through a simple optical measurement setup. Our optical measurement setup, using LEDs and photodiodes (PDs), is simple and costs little, whereas the commercial turbidity and chlorophyll \(a\) concentration sensors use expensive optical systems. Moreover, our measurement setup allows us to combine two sensors into one. Although many commercial turbidity and chlorophyll \(a\) concentration sensors require large samples or should directly contact samples due to their probe size or structure, respectively, our measurement setup does not require sample quantities that are as large, and we can detect turbidity and chlorophyll \(a\) concentration without contact. Our setup also provides more exact results in the calibration.  
In this paper, we propose a simple optical measurement method for the simultaneous sensing of turbidity and chlorophyll \(a\) concentration, and report the results of a feasibility study.