How did EG even know about the culvert? Was this premeditated, or was she driving around until she stumbled upon it?
Because it's location looks to be in a rural area, and not many people take random drives in the country. And, take mental notes of a culvert being around, unless they were planning a murder. I highly doubt that Lucas had died in his sleep unless he died from internal injuries. I think EG was high on meth and did some whacked out meth head thing. Do people hallucinate while high on meth? My brother-in-law is a meth head, but I never asked the full details about his experiences. I just know that when he is high, he isn't very nice of a person.
I believe EG lived previously in Andover, had familiarity with the El Dorado area, and may have lived in Butler County. I didn't look up Butler County on the map, so I don't know where that is. But I wonder if those country areas might be places to go for a cheap picnic or to pick berries? If she was familiar with the area, then she'd know about the stream and from that she'd be able to figure out that the stream goes under the road, so there's either a bridge or a culvert there. But I don't know if I'd be thinking clearly enough to figure that out if I was out looking for a place to hide a body.