We present an empirical study of contamination in wide-field optical follow-up searches of gravitational wave sources from Advanced LIGO/Virgo using dedicated observations with the Dark Energy Camera. Our search covered ~56 deg2, with two visits per night, in the i and z bands, followed by an additional set of griz images three weeks later to serve as reference images for subtraction. We achieve 5σ point-source limiting magnitudes of i?≈?23.5 and z?≈?22.4 mag in the coadded single-epoch images. We conduct a search for transient objects that mimic the i???z color behavior of both red (i?z?>?0.5 mag) and blue (i?z?i??22.5 mag. Using our efficiencies, we derive sky rates for kilonova contaminants of deg?2 and deg?2. The total contamination rate is deg?2. We compare our results to previous optical follow-up efforts and comment on the outlook for gravitational wave follow-up searches as additional detectors (e.g., KAGRA, LIGO India) come online in the next decade.
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