A system for sensing hydrocarbons in an aqueous solution comprising benzene, toluene, and ethylbenzene/xylene, the system comprising: a pretreatment stage comprising a particulate filter, which receives the aqueous solution to be tested and operates to provide at least a test sample; a flow cell comprising a plurality of sensors, each with a different polymer coating having partial selectivity for each of one or more analytes, the flow cell connected to the pretreatment stage and adapted to receive the test sample and a reference sample; and a microcontroller adapted to receive an output of the plurality of sensors and execute computer readable code to be stored in a data storage device communicatively connectable to the microcontroller, to process a frequency shift in the outputs of the plurality of sensors with a bank of Kalman filters to estimate a concentration of each of one or more analytes in the aqueous solution, wherein the polymer coatings are selected from poly(ethyl acrylate), poly(epichlorohydrin) (PECH), poly(isobutylene), polystyrene-diisooctyl azelate (PS-DIOA), and polystyrene-diisononyl cyclohexane-1,2-dicarboxylate (PS-DINCH). and wherein the plurality of sensors comprise: a first delay line with a first SH-SAW sensor comprising a first polymer coating; a second delay line with a second SH-SAW sensor comprising a second polymer coating; a third delay line with a third SH-SAW sensor comprising a third polymer coating; and a fourth delay line with a fourth SH-SAW sensor comprising a fourth polymer coating and adapted to operate as a reference delay line; wherein each of the first, second, third, and fourth polymer coatings have different hydrocarbon absorption properties; the system further comprising: a phase detector that receives the output signals of from each of the first, second, third, and fourth delay lines, the phase detector outputs phase shifts for each of the output signals; and an amplitude detector that receives the output signals from each of the first, second, third, and fourth delay lines, the amplitude detector outputs differential amplitudes for each of the output signals; and wherein the microcontroller is adapted to process the phase shifts to determine corresponding frequency shifts for each of the output signals and process the differential amplitudes and to estimate the concentration of at least one analyte in the aqueous solution from from the differential amplitudes and the frequency shifts of the output signals. a pretreatment stage comprising a particulate filter, which receives the aqueous solution to be tested and operates to provide at least a test sample; a flow cell comprising a plurality of sensors, each with a different polymer coating having partial selectivity for each of one or more analytes, the flow cell connected to the pretreatment stage and adapted to receive the test sample and a reference sample; and a microcontroller adapted to receive an output of the plurality of sensors and execute computer readable code to be stored in a data storage device communicatively connectable to the microcontroller, to process a frequency shift in the outputs of the plurality of sensors with a bank of Kalman filters to estimate a concentration of each of one or more analytes in the aqueous solution, wherein the polymer coatings are selected from poly(ethyl acrylate), poly(epichlorohydrin) (PECH), poly(isobutylene), polystyrene-diisooctyl azelate (PS-DIOA), and polystyrene-diisononyl cyclohexane-1,2-dicarboxylate (PS-DINCH). a first delay line with a first SH-SAW sensor comprising a first polymer coating; a second delay line with a second SH-SAW sensor comprising a second polymer coating; a third delay line with a third SH-SAW sensor comprising a third polymer coating; and a fourth delay line with a fourth SH-SAW sensor comprising a fourth polymer coating and adapted to operate as a reference delay line; wherein each of the first, second, third, and fourth polymer coatings have different hydrocarbon absorption properties; the system further comprising: a phase detector that receives the output signals of from each of the first, second, third, and fourth delay lines, the phase detector outputs phase shifts for each of the output signals; and an amplitude detector that receives the output signals from each of the first, second, third, and fourth delay lines, the amplitude detector outputs differential amplitudes for each of the output signals; and wherein the microcontroller is adapted to process the phase shifts to determine corresponding frequency shifts for each of the output signals and process the differential amplitudes and to estimate the concentration of at least one analyte in the aqueous solution from from the differential amplitudes and the frequency shifts of the output signals. a phase detector that receives the output signals of from each of the first, second, third, and fourth delay lines, the phase detector outputs phase shifts for each of the output signals; and an amplitude detector that receives the output signals from each of the first, second, third, and fourth delay lines, the amplitude detector outputs differential amplitudes for each of the output signals; and wherein the microcontroller is adapted to process the phase shifts to determine corresponding frequency shifts for each of the output signals and process the differential amplitudes and to estimate the concentration of at least one analyte in the aqueous solution from from the differential amplitudes and the frequency shifts of the output signals.