A search for a composition-dependent gravitational force

The weak equivalence principle (WEP) maintains that all objects in a uniform gravitational field fall at the same rate, and this rate is independent of composition. I have used the Northfield Mountain Pumped Storage Reservoir as a modulated source of gravitational mass and measured the acceleration of copper and polyethylene test masses towards the reservoir with a sensitive torsion pendulum. The geometry of the pendulum is such that any differential acceleration would manifest itself as a shift in the pendulum period. Therefore, the signature of a WEP-violating force would be a period shift that correlates with the rise and fall of the mass in the Northfield Mountain reservoir. I have looked for such a correlation and find that if there is a difference in the accelerations of copper and polyethylene toward the water in the reservoir, it is less that 6.0 x 10(exp -9) cm/s. We report the limits this places on the possible coupling strength of a possible composition dependent fifth force.