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We present an empirical study of contamination in deep, rapid, and wide-fieldoptical follow-up searches of GW sources from aLIGO. We utilize dedicatedobservations during four nights of imaging with DECam. Our search covered$\sim56$ deg$^2$, with two visits per night separated by $\approx 3$~hours, in$i$- and $z$-band, followed by an additional set of $griz$ images three weekslater to serve as reference images for subtraction, and for the purpose ofidentifying galaxy and stellar counterparts for any transient sources. Weachieve $5\sigma$ point-source limiting magnitudes of $i \approx 23.5$ and $z\approx 22.4$ mag in the coadded single-epoch images. We conduct a search fortransient objects that can mimic the $i-z$ color behavior of both red ($i-z >0.5$~mag) and blue ($i-z < 0$~mag) kilonova emission, finding 11 and 10contaminants, respectively. Independent of color, we identify 48 transients ofinterest. Additionally, we leverage the rapid cadence of our observations tosearch for sources with characteristic timescales of $\approx1$ day and$\approx3$ hours, finding no potential contaminants. We assess the efficiencyof our pipeline and search methodology with injected point sources, findingthat we are 90\% (60\%) efficient when searching for red (blue) kilonova-likesources to a limiting magnitude of $i \lesssim 22.5$ mag. Applying theseefficiencies, we derive sky rates for kilonova contaminants in the red and blueregimes of $\mathcal{R}_{\rm red} \approx 0.16$ deg$^{-2}$ and$\mathcal{R}_{\rm blue} \approx 0.80$ deg$^{-2}$. The total contamination rate,independent of color, is $\mathcal{R}_{\rm all} \approx 1.79$ deg$^{-2}$. Wecompare our derived results to optical follow-up searches of the GW eventsGW150914 and GW151226 and comment on the outlook for GW follow-up searches asadditional GW detectors (e.g., KAGRA, LIGO India) come online in the nextdecade.

An Empirical Study of Contamination in Deep, Rapid, and Wide-Field Optical Follow-Up of Gravitational Wave Events